There is no FTL.
Most spacecraft use He3-De fusion Torchdrives, capable of performing sustained multi-gee burns. Most torchdrives can also use fuel that is delivered by accelerators elsewhere in the system, so fuel doesn't have to be used until the actual battle begins.
Stealth exists in the setting, but it is very limiting and requires specialised rockets. It relies on cooling, anti-radar coating, meta-materials and on specialised rocket designs. Stealth vessels are generally limited to Hohmann-Transfer orbits and their effectiveness depends on the state of the sensor networks. (Interplanetary space is usually save, but going inside a well settled planets orbit is very risky.)
Automation is high and if there is a crew of biological humans present, they are genetically and biologically enhanced. Pure AI, robots or uploaded minds are more common alternatives.
While defence has beaten offence in the realm of cyber-security, breaking into the systems on a hardware level is usually possible and most factions have a numberof backdoor into each others systems. Ultimately, cybersystems have taken on a rather biological structure, with defence systems acting much like our immune systems. Data is often filled with misinformation, propaganda, computer and memetic viruses and other nasty stuff. So everything must be done with caution.
Missiles are used over long ranges (most commonly a lightminutes to a few lightseconds, though their range isn't actually limited, as they can just enter a cruise mode). Usually missiles have an acceleration stage (metallic hydrogen, fusion or antimatter rocket or a beamed energy based propulsion system). This acceleration (hot) stage is then scuttled and a stealth (cold) stage is employed alongside a cold gas thruster, which operates on evaporating liquid helium. Additionally, cooled balloons are deployed as decoys, if the missile believes, that it has been discovered. This stage is only used to avoid the attention of long range energy weapons, which could otherwise begin decemating the missile swarm from a few lightseconds away. As soon as the missiles have been discovered (either the enemy missile wave has closed in or they start taking effective energy weapon fire), they scatter their submunitions. The submunitions have been brought up to speed during the acceleration stage and focus on manovering. They fight enemy missiles and attempt to get a killing shot on the enemy vessel. Common submunitions are kinetic impactor (crude diamond blocks, but their impact energy creates a shock wave that can disable an entire spacecraft), shaped nuclear lances (small angles with huge ranges, wide ones against submunitions, additionally they work well against sensors), lasers (chemical or nuclear powered, short lived and more interesting for electronic warfare than actual damage), radio jammers, thermal particles, radio shaff and sometimes nukes for planetary bombardment.
Energy weapons fall into several categories and work best as soon as targets are about a lightsecond away. Inefficient Gamma and X-ray lasers, UV-Lasers (most commonly used and often capable of operating in the visible part of the spectrum as well, if more efficiency is required), ultra long range (several AU) laser guided particle beams and macron cannons, which fire nano-particles filled with fusion fuel at a few percent of light.
Point defense is done by batteries of masers and infrared lasers, kinetic or macron cannons are sometimes used to provide a final shield against incoming submunitions. Some navies use nuclear bombs instead. While those do indeed clear away incoming submunitions, they cause damage to the spacecraft as well.
Armor is made from carbon super materials, which are further strengthened via electromagnetic effects. Most sections of armor rotate rapidly to offer the largest possible depth against energy weapon fire. Many levels of redundant point defense and sensor systems are buried inside. Armor is usually several meters thick. Non-rotating sections are even more massive. Strong magnetic fields are often used to further reduce the effects of plasma weapons.
Radiators never truely converged on one design, however all designs today pay the energy tax of using heat pumps to achieve as high a temperature as possible to reduce surface area Either short, solid radiators are used all over the hull, operating at a few thousand kelvin and ready to be replaced several times over or particles and droplets are ejected and brought back in by powerful magnetic fields as soon as they've cooled down again. Additionally large, dumpable heatsinks are used to allow the vessels to operate even when their standard heat budget has been exceeded.
Drones are commonplace. The most common drone is arguably the missile. However, most other drones operate closer to the spacecraft itself and offer high acceleration with very limited abilities. Scouts are sent ahead and very versatile, telescopes deploy behind the battle and try to spot cold missiles (there is a trade of between keeping their spin-stabelised multikilometer mirrors save and keeping them close enough to deliver information in time.), deflectors have strong mantetic fields and try to deflect particle beams by a few parts of a degree, so it misses big static targets (they are commonly used by stations) and mirrors allow the vessel to fire its laser at the enemy from a better angle. Other designs like breaching pods exist, but they are niche systems.
Generally combat begins at about a light minute out. Missiles are fired. Pincering the enemy with missiles can be very effective. This is why a single stealth vessel can devestate a constallation of spacecraft if it can launch it missiles via gauss gun from an unexpected angle. The number of attack vectors, density of the swarms and number of waves depends on the situation. Additionally the missiles are highly modular and programmable, so their setup, complement of submunition warheads and behavior can be determined as the commander sees fit. Usually there is a lot of missile on missile combat. Either the passing swarms fight each other or an attacking swarm fights an defensive swarm. Point defense is good, but far from perfect. Once the submunitions have closed in to a distance of a few thousand kilometers, they can use particle lances and lasers to surpress sensors. Defensive nukes make trying to overwhelm the point defense ineffective, so the attacker tries to avoid offering on good target for such a sweeping defensive strike. Instead they try to grind down the defensive with an optimal number of submunitions to get at least one fatal hit (either a kinetic impactor or particle lance that will penetrate the armor). The chances of annihilation, critical damage, noncritacal damage and a perfect defence are roughly equal over all battles.
Energy weapon combat begins around one lightsecond. It is rare, as it can only happen if one combatent was ambushed or both want to engage. Normally one side retreats or surrenders after the missile phase is over. If the enemy isn't a combat craft, it will be annihilated in seconds. If both are combat craft and in a roughly equal state, the looser will take almost as much damage as the winner.
Scout drones are usually employed as gunboats.
Stealth craft either operate as spies, missile carriers (using a gauss gun to lauch cold missiles before sneaking away), strikers (which dump their stealth superstructure and trun into regular torch craft in a fight) or carries for torch craft. Stealth crafts are usually much bigger that combat craft, as they need huge amounts of coolant to operate.
Torchdrive vessels all tend to fall into six groups. Destroyers, Armored Crusiers and Battleships, which are optimised for the combat constallation. In practice most navies use just two or even one type in the battle constellation. Size doesn't translate into endurence or performance. Nukes and kinetic impactors destroy a 100m vessel as throughly as a 1000m one. Performance depends on the thrust to weight ratio.
Frigates, Cruisers and Battlecruisers tend to be more diverse and trade versatility for combat performance. They can operate independently, sometimes for years.
Size and Design
Stealth craft tend to be tapering tubes about a kilometer long, as this allows them to carry enough coolant without occluding too many stars, even while using light bending meta-materials. They always try to keep their front towards the sun.
Combat craft tend to be between 50m and 150m. This usually allows them to preamptively dodge energy weapons with their heigh acceleration. Independent craft are generally bigger and tend to have centrifuges and auxiliary craft as well as slots for mission specific modules. Depending on the type they are between 75m and 500m. Their form is optimised for the use of rotating armor, so long, outwards curved cones are the norm. The main energy weapon tends to sit in a transperent frontal cone, that is optimised to let the weapons wavelengths through. Particle beams and macron cannons use shutters.
Is this system of space combat plausible? Did I miss anything obvious?
Thanks for reading this rather long question, I build this system over a long time and am interested if it holds up as a whole.