I want to have a somewhat realistic setting where space fleets have to close in to short distances, probably around 1000 km or less, in order to inflict damage. This will be only a few hundred years into the future. Also, I don't want a direct translation of a naval fleet to space if it'll require too many justifications. I'm fine with doing away with fighters for example. What's important is that small and large ships have a reason to gather into a formation and engage at relatively short ranges.
I thought about what a setting would need to accomplish this and I've come up with the following
1) Armor, engines, and heat sinks are really efficient in this setting, made possible by unobtainium if need be
2) Lasers are out. Maybe diffraction is too high, or perhaps the armor mentioned in the first point deals with lasers very well. For whatever in-universe reason lasers are not effective
3) Missiles are the only way of striking at long range
With these 3 rules in effect what I envision would happen would be ships sticking close to one another. Small ships would protect capital ships with their overlapping point defense. With such an effective defense fleets will have to move close enough to use kinetic weapons like railguns, whose projectiles are too fast to intercept. Then, once enough of a fleet has been destroyed and their point defense compromised, the other fleet would seek to finish off their heavily armored capitals (which presumably are resistant to railguns) with nukes. All in all, a nice long fight with lots of opportunities for maneuvering.
So, what are the ramifications of my rules? Are my assumptions reasonable? Or would some other tactic be the standard? If the latter is true then what else would I have to change in order to make the premise realistic?