You'll generally get better at 6k if you play a lot of 7k, so that's what I would recommend - especially considering the much larger map pool. From my experience, 6k has a lot of rice and hybrid patterns, so BMS type charts and regular ranked/loved maps should do pretty well with getting the hang of it.
I've included links to some packs or songs that I found helpful in learning 6k, so do check them out if you'd like! (Note that some of the later ones are a little more difficult than 2* but are pretty good for learning advanced patterning.)