WRT autoremovals, made some progress but this is a bit of a hard topic. There's some breakage in the ?garbage pattern as the depcache no longer marks and sweeps itself.
Solver3 when it sees the first optional install request, marks all previously installed packages that are not yet decided as must as garbage, which is about right, except we need to solve Recommends before "kept auto package" and only trigger once we see the latter.