I think Broadcast Tools. I just got a SS4.1 for $100 from the bay. It has provision for remote switching so you can hide it.When I hear about the need for a monitor switcher, SPL products always come to mind.
https://spl.audio/en/spl-produkt/control-one/
The two 16 way switchers I have for my console also terminates each unused input with 10K resistors. Nice touch.
I recently asked Joe Malone (JLM Audio) if he would consider adding a companion board to his 8 way relay switcher for source selection using momentary action switches. He said he was developing a monitor level relay controller based on a arduino. He said programming in source selection switching using momentary action switches would be relatively easy. I can’t wait for this project but a great addition to the DIY world.