On my end, I'm also the opinion that the crossover aspect ultimately feels forced and tacked on one way or another, like you haven't given it enough relevance. Nothing in the setting given leads to or justifies multiversal interactions in any real way beyond you saying they can be a thing OOC, it reduces what you're using to pitch your game ("A Multidimensional RP") to a gimmick character select screen whereas usually dimension-hopping is kind of a big deal. It's a shame because I do think you might have something with the setting (cult trying to accelerate multiversal collapse/singularity) and more controlled non-sandbox structure.