To how disable theme updates

My site is currently on version 1.5.8 of the SKT Pathway theme, but for some reason it keeps on automatically updating to version 1.6, which breaks my site.

I made changes to the core theme files instead of creating a child theme – which I know is a no-no. I’ve made so many changes that I think it would be hard for me to go back and create a child theme now.

Is there any way to stop WordPress from updating my SKT Pathway theme files?

I’ve already added these lines to my wp-config file, but my theme is still automatically updating after a few hours on version 1.5.8:

define( ‘WP_AUTO_UPDATE_CORE’, false );
add_filter( ‘auto_update_plugin’, ‘__return_false’ );
add_filter( ‘auto_update_theme’, ‘__return_false’ );

TIA!