Are there any ships worth spending 20 AIP just for them?
Maybe you already know this, but you don't actually have to own the planet to mine the asteroids. You don't even need supply. In practice you just need to get a bunch of engies and a constructor in there and have them survive long enough to rush-build whatever you want, and then escort the spirecraft back to safety.
Actually, no, I didn't know that, so thanks. Still seems like trading the 20 AIP for a significant amount of tedium and babysitting. How's your experience been in applying this tactic?
I only recently learned you don't need supply, but I've regularly mined asteroids on in-supply, neutered planets. It's really easy and massively boosts the availability of asteroids.
For balance purposes I generally balance spirecraft as individual units because the price of the asteroid is (I think) more significant than cap of how many you can have at a particular moment. We can also look at that if necessary, though.
Per the above, for me it's generally been crystal that limits the number of Spirecraft I've built, not asteroids. The Spirecraft I use are either dirt cheap (Rams) or have high crystal cost. Adamantite and Titanite are a different story, but nearly all of my Spirecraft have been built from Reptite through Ebonite, and asteroids below Ebonite have rarely been limiting beyond the early game. (I've always had either Mark III harvesters or Mark III eco stations.)