Just saw this thread! I played the game on
PC and had a rough time learning it, then beat it pretty easily on my first run on the Switch
I got nowhere near as far as you, but found this article quite helpful:
https://www.celjaded.com/into-the-breach-squad-tier-list/
He rates Zenith Guard as High tier (only 3 squads above it: Frizen Titans, Rift Walkers, Flame Behemoths) so must think they're pretty solid! He does recommend a few pilots for the Charge Mech which might help, though (Mafan, Abe, or Ariadne).
I'm impressed you got so far, after winning I was just happy and had no desire to beat the game with all the mechs
. You've already done it with the weakest squads, so that last one should be easy with the right pilot!