My one issue is the lack of defined skills, The rogue gets some d100 roll under skills but everyone else is just left to their own devices to just play it by ear maybe using base attributes as a guide. There are optional rules for a d20 skill system but I'm not a fan of mixing d20 regular skills and d100 thief skills. I'm aware the intention of the base game is to have a GM rule based on the players explanation but I want something more robust as a skeleton to build off of.
Knave and Castles & Crusades have a system you can steal, as does Nimble 5e.
In C&C, you have a main attribute or two (or three if elf). That lowers the standard DC from 16 to 12, or something like that. They call it the seige engine. I've never played it and it was a while ago I read it so might have the details wrong. You also get class specific bonuses that are vague and situational. C&C might be worth a look as it splits the difference between 5e and old school games.
Knave is a good guide as anything that would be a skill is tied to attributes. The difference being ranged attacks are wis since that's the perception attribute. What's handy here is that Knave is classless, but each attribute is given a class as flavour text. Strength is the fighter skill, dex the thief skill, wis the cleric skill, etc. I love this system, but my players hated it.
Nimble is a 5e variant, but it condenses skills down to 10. Arcane, Examination, Finesse, Influence, Insight, Lore, Might, Naturecraft, Perception, Stealth.
Characters start with 4 skill points, and gain 1 per level.
Each skill starts at their attribute bonus. So having Int +3 would mean you start a +3 in Arcane, Exam, and Lore.
Finally I'd suggest Worlds Without Number. I've only read the sci-fi version Stars Without Number, but it's more of a houserule manual focused on world building and random tables. It's free (or you can pirate the paid version which is mostly just art) but might give some ideas for houseruling a skill system.
Sounds to me like you're dangerously close to needing to do homebrew/system surgery. Either mashing together two systems for in and out of cockpit rules, homebrewing out of cockpit rules or just straight up homebrewing the whole system. Not an enviable position honestly.
Too late for that. My go to is Tiny Frontiers: Mecha and Monsters, which is Kiaju themed. I splice it with the original Tiny Frontiers, and Tiny Frontiers 2e. All the Tiny games are basically the same with some minor re-theming. So I use Frontiers (the sci-fi game) for pilots, and the Mecha and Monsters rules for mechs and vehicles. It even has a simple way to deal with infantry. Any vehicle weapon that hits infantry instantly kills them. Clean, simple, easy to house rule around. The lack of progression and depth really limits campaigns to one shots, or short campaigns.
I've been tempted to homebrew a whole system multiple times, and release it for free, but it's a lot of work, and I can't playtest it because no one will play with me, and it would be wasted effort because no one will play it with me. And as said, I get pushback when I mention it outside of here.
It's a topic I've gone over in this thread a few times at least, and while I enjoy talking about it, I'm sure people are sick of hearing it. Though in typing this reply, it got me thinking if I can sneak it in the back door, kind of like how Eberron allows me to get my pulp sci-fi and Silent Hill fix without actually telling people that's what we're playing.
