

I’m not sure how closely it satisfies all your requirements, but I have 2 ultimate hacking keyboards, and they serve me well. Open source firmware with clients for Windows Mac and Linux. Once settings are programmed into the keyboard, they work regardless of connected OS.
It’s a 60% keyboard with no numpad… Ymmv.
Get a load of this guy, he thinks the general will be in grenade range of where the action is!