I will admit I'm not sure if I've ever heard of a split keyboard that was viewed as one unit by the host device AND that had more than two parts. I don't think the usual suspects for firmware will support that, though I could be wrong. That said, if you can live with more cords there's no particular reason you couldn't make it a split keyboard and two macro pads, or two split keyboards. To the computer, "keypresses is keypresses" for the most part. In any event, this could work out fine in several different approaches.

I'll echo @junderwood@lemmy.world and say that I don't believe there are any commercially available half-height keyboard switches with proper keycaps, though I'm sure there are various pushbutton switches that could give you the basic functionality. Still, I might write that one off for now.

For the layout, your stagger looks modest but fine; that's very much a personal thing though, and I completely borked it on my first (and only) split board. One other thing I didn't love about my split was having two vertical tiers of thumb keys. Thumbs are strong but short and move differently. Clearly it can work, but more often you'll see the arc extended rather than stacking. In any event, I'd recommend at least printing up a full-size copy of the layout and seeing how your fingers will need to move to hit everything you'd like. It's far short of a full mockup, but it would have a great value-to-effort ratio. :-)

For the numpad, I don't know your use case but backslashes aren't commonly used for math (right?) or data entry so I might move the dot/comma separator onto that group of four and park backslash somewhere else. The keymap beyond that should be fully programmable, so maybe just don't spend big on custom keycaps until you're sure you've settled in on something you like. A lot of people at !ergomechkeyboards@lemmy.world (and elsewhere) will use all blanks or blanks on the mods.