Check out the blogpost here: https://tarneo.fr/posts/split_keyboard/
Ergonomic, split and other weird keyboards
Posts must be of/about keyboards that have a clear delineation between the left and right halves of the keyboard, column stagger, or both. This includes one-handed (one half doesn't exist, what clearer delineation is that!?)
i.e. no regular non-split¹ row-stagger and no non-split¹ ortholinear²
¹ split meaning a separation of the halves, whether fixed in place or entirely separate, both are fine.
² ortholinear meaning keys layed out in a grid
No excessive posting/"shilling" for commercial purposes. Vendors are permitted to promote their products/services but keep it to a minimum and use the [vendor] flair. Posts that appear to be marketing without being transparent about it will be removed.
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Very nice keyboard!
For French, I like having a 4th alphabetical key under the middle finger (for instance, move the innermost thumb key a little upward so that is contiguous with the middle finger column). This allows having all the most common characters in the main layer (also using most of the outer pinky columns for this purpose).
Otherwise does this 2 row cluster work well for you? (In particular the upper row)
Concerning the technical trick with xmodmap, I have the feeling you are adding much complexity just for the sake of using qmk's graphical configurator. Since you are already using xkb (obviously, since there isn't any serious alternative under X11 or Wayland!), why not configure everything in xkb layout? This way you can configure any existing character without any strange workaround or third party tool! Then your use qmk just for fancy stuff (layers, combos, hold-tap, ... ).
The usual argument against xkb is that you cannot bring your keyboard around and have it ready to use on any machine. But since you are using xmodmap, I assume it is not a concern for you, is it?
You say there is no serious alternative: did you check out keyd?
Unless I am mistaken, keyd is a kernel level input remapper (like kmonad and kanata). So it is a lower level component: even if you use keyd, xkb will still be there, downstream. The situation would be roughly the same as with xmodmap... except it won't even be enough!
Indeed, this method consists in mapping scan codes to other scan codes. At this level, there is not the notion that a key produces a character yet. Hence concretely, kernel level input remapping (like QMK, btw) cannot be used to map a key to a character that is not already declared in a higher level component (usually in xkb or xcompose, but it can also be a another component using X11 API, such as xmodmap, for instance).
Edit: to make it more clear, in the end, it is the application who decides what character should be displayed. Typically, for doing so, the application relies on libraries from its toolkit, which implement either X11 or Wayland text input protocols; using standard keycode-to-character translation libraries (i.e., most notably, libxkb). To add new characters in a keymap, these characters need, in the end, to be produceable as the output of these libraries.