Okay, first to the question of disputed territories, and what level of boundaries to show at what zoom levels. These are all not any problems, because one could simply show the same thing that the Carto style layer will do… Just with thicker boundaries, that’s all.
https://osm-boundaries.com/Map looks very promising, except one, it doesn’t work on a cell phone pleasantly. Two, it’s either all or nothing: you can choose the plain boundaries layer, or you can choose the Open street map layer, but you can’t combine the two to get the heavier boundary effect.