my man!
I would propose either of the methods outlined in the initial post
For testing purposes, I have added in disable capture while building to my mod. As well as the other insanity I have been messing around with. I probably going to 're-release' my mod soon. I wish the tools were updated for which they have not for the past 6ish patches. So some stuff I either have to reimplement or do not touch them entirely, the latter is far too late.
Personally, I would say move sandbags in engies while various doctrines could add it back to main line such as Riflemen Field Defense. There are a lot of ways this can be adjusted. Did you know you can designate the max number of models can speed up production. The default is 8 and many keep the default. Example:
Two squads of conscripts which can be from 12-14 models. If both are task to build the same set of sand bags it will build at the same rate as 8 models rather than the effects of 12-14.
sbps (squad section) engineering section. Units can have different build rates as well though this is standardly default set to 1. Say if pioneers had a rate of 1.5 and are building a bunker, while grens with build rate of 1. The pioneer squad could build the bunker %50 faster or 20% faster with only 3 models.