Many orchids store their nutrients in their stems instead of the roots. I recommend not trimming anything back.

Phalaenopsis orchids are one of the very few that will bloom more than once on the same stem. I would cut the old stem back to just above the first tiny vestigial leaf below where the flowers were. Usually, you will get one or two new flowering branches from growth buds there.

I wait and see what's happening to the old stems after flowering and you can tell if it's going to flower again or if it's going to turn black or brown and needs cutting off. For the past year I have been using spagnum moss instead of bark in the pots and as long as it's kept damp all year round, only one out of nine orchids hasn't flowered continuously. p.s. I don't use spagnum moss for anything else because I know it's not sustainable.