Thanks for sharing your experience and insights on this. Migrating out of Eudora is no walk in the park! I've often considered taking the IMAP route, but Eudora's support for IMAP feels a little patchy.
Specialised conversion/migration software is probably the better route to go, especially now that the Eudora source code was donated to the Computer History Museum. I'll give Eudora2Unix a shot though!
You mentioned attachment name clashes: Eudora simply appends an incrementing integer to the incoming attachment's filename, for example, it would retain file.jpg for the first email and use file1.jpg in the next email it encounters.
FWIW it wasn't took hard to make a little Dockerfile with python:2.7-alpine and have it bind-mount the a directory of data and run in there. (With a USER directive setting it to my UID.)
Note that its "pine" output format is... almost what Thunderbird wants, except for nested mailbox effects. That'll require renaming directories from foo to foo.sbd and sometimes adding a corresponding blank foo file.