Is there a way to enable/disable PHP-FPM by default for newly created accounts?
Of course! There are 2 ways to go about enabling/disabling PHP-FPM by default.
1.) Using root WHM -> MultiPHP Manager, toggle the selector under "System PHP-FPM" to the desired setting. If enabled, this will make it so that any newly created accounts will be using PHP-FPM. You can select the specific PHP version used just above this, under System PHP Version.
2.) Through root SSH or root WHM -> Terminal, create the empty touch file /var/cpanel/ApachePHPFPM/default_accounts_to_fpm to enable PHP-FPM by default. You can remove the file to disable it. This file is created when enable PHP-FPM by default and removed when you disable it, so if you look at MultiPHP Manager after creating or deleting this file, you will see that System PHP-FPM reflects the change.
Both options do the same thing in the end, although the second options allows for scripting based automation.