Disable sync on some IMAP folders

Please see my comments here, which describe exactly how to do that:

BTW, that won’t work with a Gmail or Google Workspace account.

We also have a new feature in version 9, where you can set how far back the sync will go. So if you only want the past 3 months of your 250 folders, you can do that. The setting is in Menu > Accounts the the IMAP tab for the account in Sync Options.