How do I make prepared seitan (gluten) more tender?
Some meat tenderizers will very likely work since they are proteases, some of which work on plant based proteins too. Try on a small piece and compare.

Frying and simmering will both cook it to an even more firm state (maybe it would eventually cook to mush if you cooked it for days... but one way to MAKE it involves a 3-4 hour simmer :).

I am assuming it is not actually rubbery due to being undercooked - in that case your 1 hour braise should have improved it.