Slitherine

When I manually build structures in my regions, I always keep one slot empty and wait for the 0/-1/-2 slot buildings to appear in the browser (roads, ports, domain expansions, heralds, etc.). The automated building system doesn't do that and "wastes" all slots immediately, leaving the automanaged province significantly underdeveloped compared to the manually managed ones. As a result, I have to micromanage everything, which takes a long time for my 100+ demesne Byzantines.

It seems to me that the main issue here is the inability to construct 0/-1/-2 slot buildings in a region with no free slots. Is there any reason for this? Wouldn't it make more sense to allow construction of such structures even in a "full" region? Province management would be much easier and the autobuild problem would be solved.

It's mostly because it would require several changes, including in the UI, to still allow regions without slots to propose 0 slots or less buildings.

It would simply be to smart to be able to build 0/-1/-2 building if full.
Alternatively you can just build harbours and fortifications when you don't like the choices.