Have you seen Beancount-import (https://github.com/jbms/beancount-import)? It provides a web interface just focusing on the import and transaction reconciling/categorization piece, and includes a decision-tree-based classifier that learns as you go.
I gave it a go based on your suggestion and I found it a horror to set up (it forces you to write a new importer just for it!?) but it's quite nice once you do.
Yeah, I haven't tried it with an existing importer, but I can't say I'm surprised. FWIW, it does work pretty well in combination with finance-dl (same author): https://github.com/jbms/finance-dl
