You might already know about these, but Wilba has a VIA compatible 60% PCB that seems to be popping up all over the place.
I grabbed one at the Melbourne Mechanical Keyboard meetup and it’s really great. I used a plate, as that’s my preference, but it’s designed to have a fair bit of flex if that’s your thing.
There’s a few vendors listing it in different colours:
I am running a weird flex in my Sirius with a POM half plate & absolutely love all the flex the PCB allows for! I’d definitely recommend them for any half-plate or plateless build. If you’re planning on a full plate build I’d still recommend the WT60-D PCBs cause they are top notch, but don’t expect the flex cuts to do much unless you’re using a flexible plate material.
I think if you use a full PC or POM plate you’ll get a decent bit of flex from the PCB (these things flex hugely if there is room in the case), although I do agree the PC will be a bit more flexy than POM.
No foam, I did use a piece of paper to isolate the weight from the back of the PCB since the PCB definitely flexs enough to hit the case, but only when handling it TBH. In regular typing I haven’t had it hit off the case & I’m not exactly a light typer either LOL!