What: Have shipyards, ground force construction centers, maintenance facilities, research facilities, etc only use population when they're actually performing work. This could/should have a lower limit, e.g. perhaps 25% of max when doing no work (i.e. the workforce required to "keep the lights on") up to 100% when the facility is operating at peak capacity.
Why: Right now you can toggle on or off construction of MSP, you can use part of the industrial capacity, etc, but terraforming facilities and shipyards and GFCCs always use maximum population. This can be a strain on colonies, and right now the only way to prioritize one occupation over another is to move the facility to an unoccupied local moon. It would also add some element of gameplay choice as to how you prioritize use of the workforce when a given colony is workforce constrained. We already have wealth consumption occurring in this fashion, so why not population usage?