That being said, at what point should I try and progress up star ranks? Is it ever okay to just be fine with having a bad acc (85-90%) when moving up to higher star ranks while you get accustomed to the more fast-paced and complex patterns, or should it always just be a smooth curve from difficulty to difficulty?If you're getting less than 95% on most songs at one star level, there isn't much point moving up. You've already found a level that is hard for you to acc. If you have fun going so high you can no longer FC, then feel free.
If you want to improve, find a star level where you can FC most of the maps in 10 tries or less, then move up, if it starts taking you more than 10 tries on most maps, you've found a good level to practice at.
As for the level of accuracy you should be aiming for, you can start off on a lower target, then work your way up to 99+%. I started on 95+% and worked my way up to 99+% on songs I can FC in less than 10 tries, and at least 98% for everything else. Increasing my target has increased the rate at which I'm improving, but I know when to quit if I'm not improving my accuracy or keep failing the same part over and over. I can just come back to those maps later once I've played more.
As for the curve, I've found it's more like a staircase, you get flat stretches where nothing changes, then your ability jumps straight up and you're suddenly getting good results on songs you struggled with before...then back to 0 progress for a while...