I've done FET's 'natural' (no trigger, no pessaries), as I ovulate on my own and cycles are relatively regular. I conceived on a natural FET, unfortunately didn't work out but shows it can work.
Also have done FET's with progesterone support, reasoning being - it can't hurt. And to be honest, I do like to have the progesterone support even though I probably don't need it. Makes me feel like all bases are covered. : )