cool stuff! i think any of the two solutions you've outlined (build speed reduction, disabled capping) would be much better than outright removing sandbags from mainlines.
i was wondering if a third option, that is reducing the received accuracy and/or damage reduction bonus for sandbag cover only, would be possible from a modding perspective?
i guess something like this could be done via target tables, but maybe there is a simpler, less messy approach.
There are a lot of unused cover types which could be utilized, also it would of course means adding this interaction with every single weapon.
Aura's and Near Entities would not provide directional cover. Crits could be designated between frontal and rear of entity, assuming infantry even have rear armor, but that would make side shot very unknown.
Target Tables are likely the only way, easy but time consuming.
UI would be another problem. Would green be incorrect? Would prebuilt map sandbags count? It would probably be best if sandbags remain heavy cover.