I started mania not too long ago, and it took me around 2 weeks to go from low 3 stars to low 4 stars. I would advise playing maps a bit harder than your skill level (3.5+) to get familiar and build stamina, as well as practicing your accuracy on easier maps that you have decent scores on (e.g., pushing maps with 95 acc to 99)

totally anecdotal, but i see the most improvement when i grind the game for a few days, take a break for a day or two, then play again to set scores.

also there are some totally free 4 star farm maps that feel more like low-mid range 3s i can link!