New player advice please

Greetings! I'm currently 5 days into playing the game (played raid for many years) and was hoping for some advice from the veteran player base. I've made some pulls and (I think) gotten fairly lucky, but I'm at the point where I want to start pushing some champs to 6* and want to know who will be my best bang for the buck. I'm attempting to go (mostly) f2p on the account and want to avoid wasted effort and resources as much as possible.

Thank you so much in advance.

Yours, Thesbus