Unless you backed up you database right before the upgrade, I don’t think you can downgrade as the database schema has been migrated while upgrading to Lemmy 0.19.0.
The downtime mentioned in the README is the database migration, which can take quite some time on some instances (it took 5-10 minutes on mine).
I set up iSponsorBlockTV on my NAS: it connects to the YouTube app on your TV (as would your smartphone app) and mutes ads and skips sponsored segments.
Initial setup is a bit arcane, but once it’s done it works perfectly!