Actually doing a bit of research on Helix forum I found a solution where one simply creates a 404 page with Quix rewrite the error.php page to redirect to 404 and a hidden menu to point to that 404 Quix page.
Ref: https://www.joomshaper.com/forums/custom-404-page
Worked for me. Only problem is that error.php will be overwritten at the next template update, but I can live with that (set a note though 😉 )
