I'm curious about how tight the mounting is with O-ring friction. Excessive space for the O-rings could lead to significant PCB wobble.
I tested the dimensions for the mounting on the other board, and once it is assembled, the PCB doesn't move at all 
That's excellent news! I was concerned about potential PCB wobble, and you've addressed that effectively. A secure mounting is crucial for a stable and consistent typing experience, as exemplified by the excellent mounting in GEON Works' F-series keyboards.
However, based on my personal preferences, I sometimes find that O-ring mountings can feel slightly 'dead' or lack a touch of lively feedback. I wonder if a very slight increase in vertical space, perhaps 0.15mm to 0.25mm, could potentially introduce a more pronounced bounce and a slightly more tactile feel. This is purely speculative, of course, and I'm curious to hear your thoughts on this potential option.