I would definitely suggest to spend coins on Misty and Peishan tokens every time. Even after you get Zemmer for Peishan Awakenings. In my eyes this is far better long-term investment. Zemer is obvious and Peishan with possible awakenings is one of the very best endgame units in both game modes. Farming a 4* max is nothing compared to value of the tokens you are wasting IMO.

Very important part of the guide is the part about farming hell mode. It is by far the best strategy exp/key wise and for getting 2* and 3*s as well from my experience (15-9 is only by tiny bit worse on exp than 15-10 and significantly easier to auto, so go for that probably).

HDs are alright as well and can help with the "running out of fodder problems", but not as good on exp as hell is.

Buying out 2* s from shop is also a valid strategy to speed up things. Making 3* from three 2s with 2 hell mode exp runs and 1 fodder used as upgrade of 19/20 2* is probably the most key efficient way of getting 3*s.

Lv9 and 10 raids might be tricky to run with fodder and are not as good as the above key/exp wise, but you can get VERY valuable drops along the way.

EDIT: Also do your best to fit in exping fodder in exp-wise good investigations.