Open cities changes Bruma to be closed momentarily after a later main quest part in order to create your statue :shrug:
Actually it doesn't. OC Bruma remains open 100% of the time once you've installed it. The statue is created in both the closed and open worldspaces without having to close the city in any way.
But with vanilla Kvatch, yes as you said it would be best to only enable the open cities Kvatch once the main quest has been completed, but that is when you are supposed to activate Kvatch Rebuilt anyways.
Open Kvatch sounds like it shouldn't be as difficult to pull off as others may think IMO.
The technical details behind how KR work make this much more difficult than you may realize. The rebuilding of the city is handled in stages which are isolated in their own separate worldspaces. Since an Open Kvatch would by necessity have to all be in a single worldspace, it would mean having to literally place 5 copies of everything into the same location and setup the proper quest stage toggling to get it all to switch over properly. This sort of thing almost never goes 100% smoothly, and the CS would be hopelessly crowded with stuff since you can't turn it all off there.
Vanilla Kvatch wouldn't be terribly difficult to make work though, but personally I'd rather just avoid messing with it because of the compatibility issues it creates with the main quest. Bruma was mild by comparison.
Oh, and FYI, an open version of Bartholm was no different from opening any other vanilla city. It doesn't have staged worldspace changes to contend with.