That's it, thank you!
Bitdefenders Free Antivirus and Adguard Adblocker (also using their DNS) are installed.
I ran several tests: I started with both disabled, then running one and then the other:
Both off had the best and fastest results (maybe just a coincidence).
Bitdefender on, Adguard off, and it was the unreliable, random mess I was used to.
Adguard on, AV off, worked too, but it seemed to need some updating via internals to retrieve all bookmarks (or I wasn't patient enough).
So yeah, Bitdefender seems to be the culprit!
I always assumed it was just scanning newly installed apps and on-demand storage, so I never tried disabling it, as I saw no relation (no off switch, needs to be force-stopped in the app info).
Maybe doing a few more tries there with Adguard on and off, or some of its components, to check on my previous impressions.
Btw, sync-internals is super, but it crashed the mobile browser when trying to access the browsers settings when it was in open in a tab, no big deal, though.
AND
I figured out what was responsible for the "sorting" related crashes.
The separators I added on Vivaldi-desktop!
They don't show on Vivaldi mobile, but they count as bookmarks. Using the handy extension I have been kindly linked up with, I removed those separators, and sync the bookmark count was considerably lower when I looked at sync-internals.
Then, after syncing, the manual sorting was fine.
Thanks a bunch, everyone, for your time and efforts and of course specifically:
for mentioning AV/VPN/DNS and the extension Bookmarks clean up - Zalex
for reminding me of sync-internals that I had forgotten - mib2berlin
Much appreciated!