Mechanical Keyboards

Get any QMK board with enough keys and the other features you need, particularly if its got a VIA/VIAL config. It's inherently programmable (literally every key) and cross-platform. The "easy" answer here would be a Keychron, but there are others.

If you can drop in size just a touch, where you still have a numpad but a small number of keys are moved, removed, or resized, then there are many enthusiast and near-enthusiast boards with "96%" or "1800" layouts, the main difference being whether the arrows, F-row, and numpad are fully compacted into a rectangle or slightly separated to guide your hands.

I don't have a specific suggestion, but anything running on QMK firmware is a pretty good choice! Works on Linux, works anywhere, stores all your programming onboard so it'll carry over to other computers automatically

It's less slick than on Windows but I believe you can use those programmable keys with Solaar, if you're interested in keeping your current keyboard.

https://github.com/pwr-Solaar/Solaar

I have a Wooting kayboard. Comes with a Linux app image.

They're on the expensive side, but I certainly can't complain about the quality. I feel like this is a keyboard I'll keep for the rest of my life.

I suggest that you should spend less effort looking for special hardware and more effort learning how keyboard mappings work in your OS (e.g. [1][2]). "Linux" is a very powerful chameleon because hardware vendors almost never cater directly to that market.

[1] https://linuxconfig.org/reprogram-keyboard-keys-with-xmodmap [2] https://github.com/xremap/xremap

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.