I like the idea of a building time.

After a brief test here's a thought that came into my mind:
Would it be possible to make it so that robots can't repair buildings while they're in the "building" state? Naturally you could say that the robots are speeding up the build time, but in many occasions when using personal roboports it increases the time the player has to wait because of the extra work robots do. I found this to be especially true when building railways.

Bouncing off this maybe an option where certain types of entities are excluded from the "building" process.

The idea is good but robot that "repair " the building when you place it blocked the process . So you have a full life building that is blocked by the script , blocked by itself . Your algorithm for the building process don't take the "acceleration process" that robot make with repair kit.