Mail app Google account not syncing on MacBook Pro M3

MBP M3 currently running Tahoe 26.2. I've been having issues with Mail for a couple of months and I'm finally trying to fix it. I have two email accounts (Google and iCloud). Both are accessed on this Mac, another MBP, and my iPhone via Apple's Mail app.

The problem is specific to the Google account in Mail on this MBP- it is not syncing properly and messages in the Inbox and other folders literally disappear and reappear for no apparent reason. Sometime this happens when closing and reopening the laptop, sometimes when Mail is just sitting open with no activity. Mail is also very slow to sync up when changes are made to the Google account (like reading, filing, or deleting a message) on this device or the other devices. Eventually it all catches up, but then the problem pops up again later. Meanwhile, the Google account syncs just fine across the other two devices, and the iCloud account syncs perfectly across all three devices. I'm assuming that I'm dealing with a corrupted preference or index file, but I haven't been able to figure it out.

Things I have tried in the past few days (doing TimeMachine backups at each stage):

• Rebuild the mailbox

• Delete the Envelope Index files at ~/Library/Mail/V10/MailData, reopen mail

• Update the system (to 15.7.3 and then to 26.2)

• Delete the Google account (in Internet Accounts), reboot, recreate account, reopen Mail

• Move ~/Library/Containers/Mail/Library/Preferences/com.apple.mail.plist to desktop, reboot, reopen Mail

• Delete Google *and* iCloud accounts, move com.apple.mail.plist to desktop, reboot, recreate accounts, reopen Mail (this was suggested by a friend who's a former Apple Genius and still works in IT)

This morning I used the Find Any File app to look for other mail.plist files (any file with "mail.plist" in the name) to see if that gave me any info. The files in the User folder are all dated within the last couple of days. However, all of the mail.plist files in the System folder that all have Last Modified dates of Nov 22, 2025. I think that's about the time this problem started, but I don't remember the exact date. So maybe that's informative? Should those dates also be more current?

Every time I try something it takes about 3-4 hours to redownload all the messages off the server, and then longer to wait and see if this intermittent problem recurs. I'm really hoping someone can give me a good idea of what to try next so I'm not shooting blind and wasting time. Based on the Mail behavior (specific to one account and one device), is there a different preferences or index file I could try removing? Other advice? Any other info I could give you that would be helpful?

Thanks much!