After bumping PHP I get TypeError and deprecation fatals. Can you make the code PHP 8.x clean?

Yes. Moving from PHP 8.1 toward 8.3 or 8.4 surfaces TypeError on nullable arguments, Return value must be of type mismatches, and implicit-nullable deprecations (function f(string $x = null) must become function f(?string $x = null)). We work through the fatals and deprecation log, make the affected code strict-types clean for the target PHP version, and confirm the store boots and runs cron and indexers without new warnings.