I think applying a weighting curve instead of hard cutoffs, the way it’s done with the smaller board sizes, could work better. Something like the diagram below (excuse my bad drawing)
Correspondence games would have to have either a fixed weight from (and to) other games or something like “thinking time” recorded and used instead of the actual time controls.
Players coming back from a hiatus would have their present rank weighed less, maybe exponentially decaying over ~3 months after which they become provisional again.
IMO, only the “overall” rank needs to be shown. It’ll be less messy, and time dependant and board size dependant performance differences could be accounted for silently during rating. A 3D graph of rank vs time control and board size would be pretty cool though.
I am unfortunately out of uservoice votes, but maybe someone could suggest it at https://ogs.uservoice.com?