Are you able to narrow down the problem to the note that may be the issue, the one it's getting stuck on?
Or is there any errors or warnings in the console? How to enable debugging | Joplin In which case please share them here.
I don't assume you can share that file with 6000 notes? As often the issue is that if we cannot replicate it, it's very hard to figure out what might be the issue.
Also have you tried v2.13? It has a fix for importing ENEX files although I don't know if it would apply to your case: https://discourse.joplinapp.org/t/pre-release-v2-13-is-now-available-updated-13-09-2023/32697