Ah it seems that I misunderstood the image you provided. I have the outer thumb keys mapped to keys I don't often use (GUI and Alt) so I'm okay with them.
Maybe try looking at all the builds people share on kbd.news? I've also heard that it's not super difficult to modify open source designs and order the PCBs yourself, but I haven't tried it.
Hope you can find something that works for you
I wish I had discovered it sooner. I work with a couple mechanical engineers and it feels like they come up with this sort of things daily