i'm currently getting better at oni's and it's really a tough step in my opinion. from my perspective muzukashii's are very similar in difficulty. there are some faster and some weirder ones but ultimately once you got them down they shouldn't be a problem anymore. on the other hand oni's are very different. there are 3* onis, 4* onis even 5* onis. but star rating also starts getting really trippy in comparison to the beginning. so it might be hard to get into it.
if you currently play maps that have like lots of triplets of the same colors and lots of single notes then you should start doing some maps that have patterns with aab or abb.
https://osu.ppy.sh/beatmapsets/110292#taiko/303142 this was a extremely great gateway oni for me since it has some of these patterns but just a few sprinkled in there
so try playing both muzukashii and oni a bit and just have fun with it