You are doing better than I am if you are running J!3.10.12 on php7.4. I'm running on php7.2 and when I try and update to 7.4 I get an error message without any details. I don't think that J!3 supports php8 if I remember correctly so you will have to upgrade to J!4. Good luck and I hope you get it figured it. Please let us know if you.
I finally got my J!3.10 to upgrade to php7.4 and even to php8. But it was not easy and something broke Joomla when I finally upgraded to J!4.
In order to do this, you have to make sure that you are using the latest version / last version released for J!3. Some developers made sure that their software was compatible with J!4 so it'll work on J!3 & J!4. Others made a completely new version that only supports J!4 & J!5.
Alright, so once you've confirmed that all of the software is ready to go, next check your htaccess.txt or .htaccess file. I had something in mine that talked about 7.2 and so what I did was delete the whole thing. Once I did that I was able to update to php7.4.
During your software check, make sure all of the extensions supports php8., if you are in doubt, disable or uninstall it if you can. Once everything is disabled or uninstalled you should be able to upgrade to php8.
If after following the above, if you are able to upgrade to php8, start slowly re-enabling your extensions. If you enable an extension that breaks your site you will have to downgrade back to 7.x re-disable the extension and upgrade back to php8. Then of course find out why that extension broke your site.
I hope this helps anyone who is looking to ugprade