The fulcrum comes to mind: It has a 5-way joystick (up, down, left, right and push) for the thumbs. You can always add tenting if you want (some alternatives)
You also can check some of the split kb databases if someone created something similar.
If you haven’t decided yet, I find ZMK is a breeze to get the running
It is. The rabbit hole is deeeep.
Next to U4s I can also recommend Durock Shrimp Silent and TTC Blueish White Silent
I think this is Colemak DH Layout. If you want to give it a try there is a training site for it - no additional hard- or software needed.
You program/configure your keymap into the keyboard’s firmware so it is independent of your operating system. If you use Mac, you put your Mac specific keys into the map where you want them. It’s totally up to you - if you want every key to be “a” you can do that :)