The Tach drone and the decloaker are the same ship. They fulfill the same role, and therefor one of them should really be removed and the remaining one changed to fulfill the intended role. That's my personal opinion at least.
I think the current structure (but probably not the current numbers) actually makes sense: you start with the "free" ability to cover X wormholes with tachyon beams, and you can spend K to unlock the ability to cover Y more.
There are other unlockable sources of tachyons, and there are bonus ship types that provide it too, but each has their cost.
Of course, that's speaking only of the defensive static use of a tachyon emitter, in which case the tach-drone/decloaker are just redundant (conceptually, not cap-wise) with the tachyon-beam-emitter/stealth-tachyon-beam-emitter.
The difference comes in their mobile use. And there, honestly, I don't know how useful they are. Scout Starships seem to thoroughly outdo them in terms of the "tachyon ship to bring with my fleet" role.
Frankly, I'd be ok with just removing both the tach drone and the decloaker and increasing the cap on the tachyon-beam-emitter and stealth-tachyon-beam-emitter and calling it a day. Maybe removing the tech prereq on the tachyon-beam-emitter (iirc it has one, I may be misremembering).
Or we could do that but bring the Decloaker back as a K-unlocked AIP-free tachyon warhead (keep the tach warheads as the K-free option).
Or maybe something else. In general I don't see any obvious "make these things more useful" approach that doesn't get unbalanced (giving them like 40,000 tachyon coverage), silly (giving them a bunch of other abilities), or overly complex (giving them some kind of auto-hunt-cloak-signatures behavior).