Moving a large and old codebase to Python3

Many of your observations feel like lurking bugs in your py2 which only exposed because you moved to py3. Others felt like the kind of design decision you regret irrespective of language binding, like depending on an externally sourced component from a company with bad customer engagement.

I still struggle with u'string' and use of encode() and decode() in the presence of input streams from the web which don't identify encoding, are probably Microsoft isolatin1 and get uplifted into UTF-8

I believe your code would benefit from py3 typing but have no personal experience of it: I just observe many of your problems felt like weak type binding problems across boundaries.