What about just buffing the SACU rapid fabrication by increasing both the mass cost of the upgrade and build power, so it becomes a viable option on those maps where hives are good - i.e. it gives concentrated build power that doesnt cause pathfinding woes (since you wouldn't need many of them). Since you'd get a mobile unit with a basic combat ability it'd need to be slightly less efficient than a hive at building, but not much.
For example, instead of a modest increase in build power from 56 to 98, quadruple the build power from 56 to 224, and also increase the build range slightly. Then set the mass cost of the upgrade accordingly to balance it. E.g. a hive costs 350 mass per 25 build power. Since the value of having a mobile builder with combat ability and basic resource generation is priced into the base SACU model, lets make the rapid fabricator upgrade as efficient as a hive in respect of the increased BP element. So taking the Sera ACU, and assuming it's roughly balanced at 2k mass for 56 bp, that'd mean you'd want the extra 168 bp to cost 2,352 (total cost of 4402 for 224 build power). By comparison, the current upgrade gives you 42 bp for 800 mass (so is significantly more expensive than a hive in terms of the additional bp).
If this would be too powerful, then make it 10% more inefficient for the additional BP.
Personally rapid fabrication feels really weak and not worth getting (I'd rather have 3 Sera SACUs than 2 rapid rabrication Sera SACUs, even if I'm planning on using them to say build a firebase or setup some SAMs) and I've never seen anyone get rapid fabrication SACUs, so it feels like an opportunity to increase variety a bit, it preserves the uniqueness of the hive, and it doesn't require work with creating a new unit.
