Yes, I can change the max_input_vars, but if you change the PHP version the max_input_vars will go back to the default value. Therefore you should make sure you're using the correct version of PHP prior to changing the max_input_vars. Generally it's a good idea to use the latest version of PHP that your software supports. Since Moodle doesn't support PHP 8.5 yet, that means PHP 8.4 is the latest support version, and yet you're using PHP 8.3.
Like I said, is there any particular reason you can't use PHP 8.4?