I have now submitted an update for the product (sadly only UE 5_0 and newer). And oh boy, I must say.. The BP was not great as it was. To put it nicely it is nice to see how my own knowledge of the engine has improved over the years 
Well.. live and learn.
Anyway. Now (as the update goes through) the bp is as it should be using Hierarchical instanced static meshes. I also did overall cleand up and simplification for the whole BP and also combined all the functionalities (constructions script and creating buildings in game with event graph function) into one BP.
Performance was greatly improved and the whole thing is just cleaner and nicer.
Cheers!