vivoactive 4 (all versions) SW 5.10

Will I have to reinstall all installed apps after reverting to 5.0? Or is it enough to make a backup of the *.fit files in the Garmin folder? Sorry for this stupid question :/

No it's not stupid. After all, Garmin should make this a little easier considering just how often one feels the need to restore to factory settings...

The only "Apps" you can recover are the default sport ones that you have configured with specific data screens etc. These are found in the Sports folder. 

Here is a link to Garmin's backup and restore instructions...

https://support.garmin.com/en-ZA/?faq=hWholZ8uPZ6PboXu5Nf7d6

IQ apps, widgets, faces, etc, effectively need to be reinstalled. 

Also, before you reconfigure everything after a successful downgrade to 5.0, is give the device one complimentary forced restart (hold down top buttons for 15+ seconds) Also, you'll need to allow Express or Connect to download the new 5.1 update again, after which your watch will notify you of the update which you dismiss. After this, you plug in your watch to your computer, copy the update file off the device, open it in a simple text editor, clear the contents and save it. Copy that back onto your device. Your device will not update to 5.1 after this. Just don't sync with Express again, as I've seen this overwrite the file again, however, syncing with Connect app was normal and didn't re-download the update. (my experience so far with the downgrade)

I don't based that on a single factory reset. I'm on the 4th or 5th factory reset at this point and I've rolled back to 5.0 as well and the issues have persisted. My assumption that something "irreversible" happened comes more for the fact that a roll back solves nothing. But that's just an assumption I have no idea how Garmin bundles all there firmware's.

Ok, and when you rolled back, and factory reset your device, does it persist even if the watch is only left in it's "stock" configuration with no IQ apps?

Is it a battery drain issue? If so, does it go away if you disconnect your phone on the watch's control centre? Some watch and phone interfaces can be strangely problematic.

Email the beta team, which I've linked above somewhere, and send them a list of things you've tried. They seem to be more helpful than the useless support team at the end of the telephone or email. 

I work in the robot business, and I have noticed that some nodes can not be downgraded. They will only accept firmwares with higher version numbers, not lower. Maybe some Garmin modules work the same way.

I have the feeling with me it is also mainly over night. Fully charged before going to sleep, 82% battery when I get up.After getting up after 5 hours about 3% less. During the day everything runs more or less good.

Same issue here, 5.1 is draining the battery too fast even over night with no SpOx measurement. With 5.0 Battery lasted 4-5 days minimum, with 5.1 and same usage 2-3 days. How can I get back to 5.0 ???

Read my posts above. I've explained how to roll back

Came searching for an answer to my massive overnight battery drain all of a sudden. Glad to see others are having the same issue. No problems previous to this update.

What's our current solution? Trick or into a downgrade? That almost seems like the better route vs a factory reset that likely isn't going to help?

Different people have had different results. I personally did the reset, did not help, did the rollback but then incorrectly stopped it from upgrading again - and it it upgrade again. But now - still on the new firmware - and the battery drain seems much lessened. Still a little more drain than pre update - (all happening overnight) - but at the moment I am not changing anything.