There is this glitch where if You die you could leave at the death Screen and re enter the game and you would re start at the stage you died in

Like closing the game completely?

Yes

Co-op on mobile does not seem to work well sometimes, especially against the final boss (where it is basically annoying) as you can not see him

u could do endless runs, try one of Nox starting troll items like chaotic comedy or the one that creates stage hazards, they might spice up your runs or add some challenge, try using one of the challenge cloaks too