If you have access to telescopes, reconnaissance platforms would be invaluable. That might be too much for your technology level, so instead scouts might dip briefly into the atmosphere, buzz your enemies, and yeet back into space without landing (If this is within the capabilities of your ship).
Dropping rocks from space would be a devastating attack, as would straight up crashing spare ships into enemies. If navigation crystals are not prohibitably expensive, someone has probably done this. If you've got parachutes, you've got paratroopers (this may again require the ability to dip briefly into atmosphere without landing).
The biggest effect is the ease of planetary transport. Once you're in orbit, landing anywhere on the planet takes about the same amount of time and energy. If you're a big naval empire, you might leave a pile of troopships in orbit to drop in and reinforce any of your colonies at a moment's notice. You'd also probably focus more on using ocean-faring vessels to defend or attack ports you care about, in order to protect your troopships as they land, or keep enemy reinforcements at bay. If two naval empires were to fight, they would probably focus on holding the important ports with your standard seafaring navy, and if possible holding the space around the planet itself. If a smaller country were to fight a spacefaring one, they might focus on inland targets, or use hit-and-run tactics to flee before reinforcements arrive.
The rest of how strategy and politics would change with spaceships depends a largely on how powerful you are willing to let ship impacts be.
If you go off the physics, hitting a planet you don't like with a chunky startreader at even a sizable fraction of the speed of light would be enough to render large portions of it uninhabitable for a couple dozen years (or maybe even crack the planet..). If you allow this, it gives every person with access to these crystals the equivalent of a nuclear bomb. Your nations would exist in a constant state of mutually assured destruction, and your crystals would be a very tightly controlled resource-- can't let any potential dissidents get a handful or they might nuke the capital from orbit.
If you don't find this state of affairs very interesting, perhaps ships phase out of reality when they're traveling faster than light, and so ship impacts are considerably less dramatic.
Either way, control of the space around your planet is critical for both strategic and mercantile reasons (can't do interstellar trade if your neighbors blockade you or charge ludicrous tariffs). If at all possible, people will be developing space combat systems, even if its just pulling up next to people and shooting them with crossbows.
One final note is that large lakes would become a lot more politically significant. Without startreaders you couldn't really go anywhere with them, but now they give you the same amount of trade access as the actual ocean, plus risk of invasion. Buy waterfront property now!