There are any way to do that automatically?
No.
PHP-FPM is your answer
Why?
Your first post shows that you know a bit about how these things works. That's why he gave you the short answer.
The long answer:
1. Install PHP 7.3 through PHP-FPM.
2. Go to Webserver Domain Conf and choose the domain(s) you want to stay with 7.3.
3. Select the appropriate Webserver configuration and then tell the panel to apply PHP-FPM 7.3 to those sites.
4. Double check everything is working right on those domains.
5. Now, upgrade your default PHP version to 8.x.
Now all websites will be using PHP 8.x except those domains you told the panel to keep using PHP-FPM 7.3 at point 3 above.