macOS Ventura Apple Mail messages are blank / empty

After reading and testing some possible solutions - including switching to outlook - I seem to have fixed the issue (for now). I exported all my local inboxes to mbox, deleted completely the ~/Library/Mail folder, then went to the web-version of my work email and gmail, deleted as many as possible email (went from some 70k back to ~6k emails, including sent email copies) and then opened Mail again. It downloaded everything from the servers and then I added back my local mailboxes where I archive my mail, importing back the mboxes.

I haven't run into blank emails yet and also have only sporadically the stuck "Downloading messages" notice.

Before this solution I tried rebuilding the mailbox and lost several emails, so I do not recommend that.

Whether this is a general solution, I really do not know. It does seem that a corrupt mail screwed up Mail's indexing, like in the post below, but I could not find out which email, so I tried this more rigorous solution. My problem may in part also be due to large number mails that were in all mailboxes, but still I would expect Apple Mail to be able to deal with that.