Mattermost upgrade 7.1.2 to 7.8.2 fails

Any hints, how to make upgrade mm 7.1.2 to 7.8.2 work. See Upgrade from 7.1.2 to 7.8.2 ESR fails · Issue #22874 · mattermost/mattermost-server · GitHub .
Wondering, whether I should remove the misbehaving focal bloat …

Removed focalboard, playbook and calls plugins incl. related tables, so that no migration errors get logged anymore, but still many files aka attachments are not shown/can not be accessed in the UI anymore. 7.8.x seems to be really buggy.