Hello!
I was looking to another stuff and found something in the plugin code that might help.
$givingLevels = apply_filters ( ‘seamless_donations_giving_levels’, $builtinGivingLevels );
It seems that instead of using add_filter( 'dgx_donate_giving_levels', 'my_custom_giving_levels' );
you should use add_filter( 'seamless_donations_giving_levels', 'my_custom_giving_levels' );
Anyways, I’m not sure and have not tested. Just sharing something that might be of use to you.