Fix plugin conflicts and errors on wordpress website

I have had other freelancers do work on the website but unfortunately they did it on the main theme and not the child them, so I can't update the main theme now without messing up the whole website. I have had to restore the website from backup multiple times now so want to avoid doing that again. I have plugin conflicts and deprecated PHP code is causing a HTTP error also (see attached image). Domain is www.ink-illusion.com. My Elementor editor isn't working also and this is all related. I need someone to set the website up on staging and clean up the entire website, making sure I can update Wordpress version, PHP version, theme, and plugins moving forwards without any issues. This means finding any customised code on main theme and moving to child theme. Also I believe the child theme may have woocommerce code which is out of date, so that needs to be moved to main theme. It is all a bit of a mess causing many problems.

Many thanks