Can the developers please add some apply_filters() calls in the code?

The easiest ones to add is in the functions in src/Infrastructure/WP/Translations/BackendStrings.php and src/Infrastructure/WP/Translations/FrontendStrings.php

For example, change:

return [ ..... big array ];

to

return apply_filters( 'wpamelia_backend_common_strings', [ .... big array ] );

I know that the strings can be changed via .mo/.po files. I only want to change one string "On-site" ('on_site') to "In person" or "In studio".

I would also love to be able to disable on-site payments if the user is not logged in. I might use the filter in WordPress's get_option() call to do this.

I did write some custom code and it is working for me. It's a bit of a hack because it's changing the JSON string.

add_filter( 'option_amelia_settings', 'rtp_disable_onsite_for_bookings', 10, 2 );
function rtp_disable_onsite_for_bookings( $amelia_settings, $option_name ) {
    if ( is_admin() ) { return $amelia_settings; }
    // Disable on-site payments unless logged in. This forces customers to pay via Stripe.
    if ( !is_user_logged_in() ) {
        $amelia_settings = str_replace( '"onSite":true', '"onSite":false', $amelia_settings );
    }
    return $amelia_settings;
}

Thank you for the information about customizing the booking form labels. I will definitely use this.