workaround 3 should not have negative consequences, but a backup before can’t hurt. The process is described at Process old emails once again
Basically its export a new backup, open it with SQLite Browser then change the uid validity value of all folders:
config table “imap.mailbox.INBOX” and “imap.mailbox.DeltaChat” set both to 0:0. (if the number before the colon is already 0 change it to sth. else)
then reimport the backup and wait until new messages are fetched.
[if your using a brand new version the backup might already be a tar file, that contains the sqlite database]