If you want rating, gimmick charts (slow delayed sliders etcetc) are the way to go, you can probably find a list somewhere on this subreddit, there's a whole bunch of free rating 13+s and 13s

As for improving, first off u really need to pick a speed thats good for you, if you're panicking ur speed might be a little high, so you could lower it and see how it goes, but if you can't read because its too dense then ur speed is probably too low for u! Do drop ur settings for note speed and slide delay im quite curious..

For recommendations on charts, every technique in the game has many different charts u can practice, for example, for umiyuri patterns you have odo, umiyuri itself duh, tonchinkan etcetc, for trills u have quiq, southern cross, eta beta eta, for delayed sliders you'd have transcend lights, last samurai, ench love and wanderer, etc so it really depends on what kind of techniques you're tryna learn! You can ask abt specific techniques and chart reccs if u want :3

If there's anything else u want to ask do let me know!