@All that’s the difference between having php as php module vs running it as extra service that needs to connect through a socket or port.
mod_php integrates php into apache, that’s why you then can control settings from .htaccess for instance.
There are other drawbacks instead, like only one php version can be run as module and you’ll have it in every apache process spawned etc.
As mentioned above, with php-fpm you can either create your own templates to allow different settings or use a .user.ini per web-page, maybe start here to get the idea: https://www.gearhost.com/documentation/how-to-configure-user-ini
You probably could deploy something preset into the users public-html and have it owned by root, if you’d want to protect it from changes, however be aware that some software automatically can detect the way of php is connected and may want to create it’s own .user.ini files (wordfence comes to mind)