Planetary defenses have infinite power available, no mater what they mount, they have the power for it. Effectively each weapon has its own reactor attached to it. So why do planetary defenses have such a limited amount of missiles ? At the least I would expect each missile bay to have the equivalent of an Extra munitions modifier by default.

I agree that ground bases should have unlimited ammunition available, not least of which because they can't mount extra ammo.

And, perhaps, a reload of Small craft after some few turns?

I'm not sure where fighter "reloads" would come from -- I don't think that new fighters can plausibly be built during the course of a battle, and if ground bases had more fighters they would use them rather than hold them in reserve. But I do think that small craft need to be more survivable in general, and not just behave like more expensive missiles. Both fighters and missiles need significant tweaks.