I keep my hands at shoulder-width and put them on the desk in a natural-feeling position. Wherever they land, that's where I put the keyboards.
Not sure if there is a better appraoch to place them in the best ergonomic position, but my wrist injury has gotten much better lately. That may just be a coincidence, but it feels like it's at least safe to say what I'm doing isn't making the injury worse.
Does that require any oddness when it comes to modifier keys like shift, ctl, cmd?
I experimented with double keyboards and thought it required the modifier key to be used on the same keyboard as the other keystroke, which basically raised the bar too high for me.
I expected those kinds of issues, but it actually works well!
So I can just do left-shift on the left keyboard, any right-side letter on the right keyboard, and it will be uppercase. Same with ctrl/alt, etc.
I can't promise you won't run into something that doesn't work, but I haven't found such a thing. On macOS my Karabiner keybindings also work without needing modifications to the config or whatever.
You can find bespoke assembly services with a lot of the vendors, it will usually run around 100$ extra so 300/400 for the keyboard which altogether isn't that bad compared to prebuilt ergos like the Ergodox.
These tend to be location specific but try to find regional resellers of the PCBs.
I bought mine on eBay, but I also have used https://boardsource.xyz for parts. They have assembled versions of both Lulu and Lily58 pro for sale, which are very similar to the sofle as well as some 40% options if you want even more minimalism. There's also the mainlander: https://www.zsa.io/moonlander/, but it's a heftier size and price tag.
https://willfennel.com/posts/2022/10/26/cheap-ergonomic-keyb...