Got a setting I've mentioned before knocking around, that of the Moon Marines (actual military title pending, but they're space marines who's primary base is on Luna). Given that most campaign ideas revolve around the party actually playing moon marines and romping around the universe breaking things, or playing the security/away team for an exploration/free trader ship, gear's going to be a thing. In this setting, humanity is also substantially more militarized than most of the galaxy (everyone else hit the point where war became too horrible to contemplate and actually stopped, or are super honorable/gullible, or can't utilize the FTL system without crysosleep because they get incredibly nauseous and/or horny, and other such things), which means their ships need to be durable and such, and at least the basics of a space-military need to be determined. In attempting to pick out various units for this approximately TL9 setting, I made a discovery;
Ultra-Tech, which I've glanced through before, is kind of terrible if you're actually trying to use it for things with any depth. Also, approximately half of TL9 is just stuff that we have today at least as prototypes, although it was still futuristic when it was printed. Also also, basically the entire conventional guns section is errata'd, and a great deal of other stuff. It mentions that spaceships are a thing that exists, and kind of sort of gives some examples, sufficient as long as it's merely a setpiece for the party to glance past on their way elsewhere.
Spaceships has some nice depth with reasonable simplicity, but a couple things that seem off. I can't tell whether armor and weapons are balanced, or if weapons are overpowered. Damage comparison between the 100mm cannon in spaceships and ultratech are kind of weird (the warhead and projectile impact damage seem to be switched, approximately, although the damage in the table may be meant for beam weapons or just an approximation with guns using the warhead table exclusively). Decade scale makes things weird. Assuming medium or secondary batteries are "standard", it takes approximately 2 of your available 6 systems per section of the finest armor available to be able to soak an average hit,
GURPS Traveler seems decent, other than being for a specific setting. It offers better ability to armor up a streamlined ship and such, at cost of marginal requirement for calculations.
I've even busted out GURPS Vehicles to look at that for comparison
I'll probably just wind up building the ship in every system and bashing them together until I get something I'm happy with. Then I just have to fill it with crew, because it's small enough and going to be the main setting enough that it's probably a good idea to have at least a name and basic personality for every crew member.