flatbield

I like Zim. Used it for years. The big advantages you can have many 1000s of pages and it just uses a folder tree not a database, so you have direct and attachment access if you need it. Zim is a true hierarchical wiki not a simple notes app. There a plugins you can enable for more advanced features.

Zim does get slower with more pages for some operations like searches and some changes. I have one wiki with 4500 pages and do feel it is getting a bit slower sometimes. You can however just create another notebook at any time as long as your content has reasonable dividing lines.

Even if you say to keep a local copy, I do not think it is truely an independent local copy unless you actually move it to “Local Folders”. Otherwise it is probably just a local replica.

No expert with IMAP, though I have used Thunderbird for decades, I have just switched to IMAP myself in the past year.

POP3 or IMAP? Desktop or Android?

If IMAP, are you sure your local folders are actually local?

Edit: By local I mean in the “Local Folders” tree not under your domain tree where the inbox is.