Of course, there are still some unresolved issues, such as the fact that it’s still impossible to create external connections for complex-shaped ceiling cornices.

This issues was (partially) solved. Read this posts:
Baseboards (first attempt).
Moulding plugin (second attempt) and Moulding furniture library .
and the final attempt:
Moulding Boolean Convert plugin (MBC) , last alpha version.
It compute corners as geometry but not the texture (I'm not good in this part).
You can design and use your own borders, provided you follow the rules outlined in the first post.