Is f2p actually feasible long term?

I've only been playing for just over 2 weeks and I really am liking the game. My aim is to be mostly f2p. I don't mind making a purchase here and there but my budgetary constraints don't allow much spending. I picked Athel and got her to 6 stars pretty quick. I have been farming brutal 12-3 a lot and leveling food for my next 6 star champ. The issue is I am consistently out of energy and am getting really fed up with spending my gems or just closing the game while energy refills. I've heard energy availability gets better later in the game but, ngl only being able to play in short bursts feels bad and has me wondering if f2p is even really viable without horrifically slow progression.