https://wiki.ai03.com/ is what a lot of people use. Though it's probably easier nowadays to use an RP2040-based ProMicro form factor daughterboard so you only need to handle key placement.
As one of the QMK Firmware maintainers, it's great to see you're going ahead with the customisable side of things even on the input side.
Might I suggest engaging with QMK early so that we can avoid the usual manufacturer "hey can you merge this, we've shipped already and forgot to raise the PR until now"?
None that I'm aware of. And I don't plan on running one of my own -- too many other things going on.
That said, I ended up creating a Discord months ago after a whole bunch of people were going to run their own GB, needed a place to "host" it... never eventuated due to the COVID logistics problem.
Only a couple of parts are actually dependent on the side you're soldering (and are listed on the parts BOM), virtually everything else is aligned in such a way that it can be soldered onto either side.
In a week or so, QMK will pick up a whole bunch of new support for pointing devices -- Djinn's open-source, so if you're keen you can swap a display out for a Cirque touchpad for instance.
Right now, none really -- it's a hardware enablement testbed, and really only shows a handful of information, such as Caps/Num/Scroll and what layer you're using.
With QMK we get a lot of requests for "how do I display this", "how do I show that", so it's more a case of making sure it supports most of those scenarios.
Sadly with the silicon shortages, not a lot of people are getting the chance to build them... I'm sure we'd see some interesting usages if/when this display stuff becomes a bit more mainstream.
Ended up designing a 64-key one with displays: https://github.com/tzarc/keyboards/tree/main/Djinn -- can't see myself going any fewer than that.