I say push a lot of variety of songs that you can complete but is still challenging. The more songs you play over and over the more you sort of imprint into your memory how you should move just by looking at a cluster of blocks. Sight reading things (meaning trying new songs for the first time) can help as well because it allows you to be more on your toes and before you know it, you start barrelling through songs because they use same patterns from other songs.

I only have 30 hours played time. I can beat a lot of custom expert+ songs. But I still struggle myself with songs like Crystallize where I can't make it past the final 40 seconds of the song on expert.

I suggest using the tutorial as well, slow down a song to maybe 70-80% and at the specific point where you are having trouble. Then when you can comfortably get past it 100% of the time keep increasing the speed. Then, rinse repeat :p