So you have some modules that don't play nicely with php8 too.
Install thirty bees' Collect logs module and look what it gives you as info on 7.4, fix some of those and try again.
The update method is as follows:
1. You are on 7.4 and your site runs without notices, errors, warnings, exceptions, etc.
2. You update using Core updater by pointing it to target version.
3. Your site is now broken because some dependecies don't work on php 8
4. Your switch your server to php8
5. Your site is working again because at step 1 you fixed all stuff that thirty bees has no control over (external theme and modules).
If you want to proceed updating to php8.1:
1. You are on php8 and your site runs without notices, errors, warnings, exceptions, etc. - THERE WILL BE NEW ISSUES in Collect log module 100%.
....
proceed according to the list above.
If you want to downgrade and not use a backup and your BO works after the update of php version (not your case obviously):
1. You are on php 8.x. and your site works
2. In Core updater you put target version 7.4.
3. Update dependencies with Core updater
4. Your site is broken.
5. Downgrade php on your server to 7.4
6. Your site works if it was working before on php 8.x