We have been trying to stamp out reasons to fast forward the game for quite awhile now.
Even on 8+ people who play for maximum efficiency wind up having to wait on fleet rebuilds (I think the need for fleet rebuilds is fine, the time cost is part of the game, the problem is that the player often has nothing better to do than wait during that time). Edit: actually, the waiting is far more common than just 8+ and maximum efficiency, that's just where it gets more noticeable iirc. Netflix queues shouldn't have to be part of your AIW strategy.
If the shields regen'd during that time it would impact their balance significantly.
I do realize that it's psychologically difficult to deal with a completely unrepairable forcefield unit, but that's what makes them balanceable: the same thing that makes martyrs balanceable. I wouldn't say martyrs are actually balanced, they're in the upper range of "OP but in a hilarious way", but they would be completely coocoo-for-cocoa-puffs if they weren't one-use and were repairable.
Using the spirecraft shield bearers for normal FF duty isn't a very good idea: in those roles you need sustainability. The spirecraft ones are for critical situations where you don't need to get through the same challenge every 15 minutes, you need to get through this
one. Like the AI EMP'd your homeworld, or you're attacking an AI homeworld without supply, or you're trying to hold a wormhole against a particularly nasty exo-CPA-wave-hybrid combo, etc.
I'm quite happy to adjust their HP and/or asteroid costs further (and we could maybe laterally-think-up some extra things like protecting things under it from sniper, ion, tachyon, etc), but the one-shot nature of them is basically the defining characteristic of the unit, I've not been able to see any way around that.