Alternative: make the drones a standard part of the T2/T3 engineering upgrade. Not targettable or selectable, just extra ACU build power. Basically just fancy visuals, like Cybran engineers' little drone things. (Practically speaking, this is just removing the upgrade but keeping the graphics.)
Another alternative: as above, but as a separate build-power only upgrade. Possibly T2/T3 engineering could be cheaper with less BP. Affects balance since e.g. TML upgrade is less useful without build power which doesn't work if both are back upgrades (could drones be moved to a 4th non-conflicting category?).
@FtXCommando's suggestion of radar is interesting, though I wonder if they'd just get killed by inties (making it only useful if very cheap, and making the second drone useless). ACU radar drones which can't be shot by air might work though might be OP. But inties already can't shoot radar buildings so maybe it's fine.