- portable, small (60%) keyboard
- Must have split spacebar [...] configurable so that I could make one of my thumbs the Shift key.
- If staggered, there must be a key to the left of Z (ISO stylee) at 50% offset.
- Must be able to move the keycaps around so that I can set it to Colemak.
- Not overly expensive (maybe $150?)
I haven't noticed such a mainstream keyboard.
This is a set of somewhat non-mainstream criteria. Ergonomic keyboards are a niche, meaning they have smaller production volume and therefore higher per-unit production costs. For a vendor to make any kind of profit, they need to price niche non-standard mechanical keyboards in the $200–300+ range (like ErgoPro, Kinesis Advantage, Truly Ergonomic, Ergodox, or the upcoming keyboard.io), or else figure out a way to hit higher volumes without spending money (unfortunately most of the ways of getting new customers involve spending a lot on marketing, and those costs show up in the final price).
(The Diverge is a different story, since the guy making them seems to not really care about paying himself a livable salary from it, he’s just one guy not a company with employees, and he’s working at a very small scale [I think he’s a college student?].)
Anyway, the most reasonable “mainstream” options for you are probably the Kinesis Advantage, the ErgoPro, or the Truly Ergonomic. All three are $200–300, and none is quite as portable as you’re looking for. They all use sculpted keycaps so if you want to have Colemak legends you’ll need to add your own aftermarket keycaps. I’m not sure if any of them makes it easy to configure one of the thumb keys to be shift.
If you really want to get something that very specifically matches your criteria, I think you’re going to need to build it yourself. If you have access to a laser cutter and a soldering iron you can probably make a layered acrylic keyboard in the precise shape you want, and get the whole thing down to ~$100 + a whole bunch of labor. Or else pay someone to build your design for you, but that’s probably a lot more than $150.