Newish player advice

I've been playing about a week and have turned in several times. Currently around stage 750 and progress is slow. If I turn in now I can get 5k stars. I have 400 candy and I'm wondering if I should save for cosmic mushroom and then turn in or upgrade special cousins. All special cousins are unlocked and currently lvl 400. Also worth mentioning, I have trumpet lvl 1. Is it worth upgrading to lvl 2? Thanks in advance!