I’ve been using a Sofle split for almost a year, probably in about 30-40% of my typing. Despite tweaking my setup as best I can, I still find the experience difficult.
One issue that seems to have a big effect is that I still think of the position of mouse in my dominant hand and keyboard with my other hand as useful.
I use it often for everything from casual surfing to editing. For example during editing you’re often selecting text with the mouse and doing some minor editing with your other hand. Split keyboards seem to really remove this efficient option since both your hands need to be used most times.
A lot of people who extol the benefits of split keyboards are comparing to traditional keyboards when your tasks are static.
A split, 2 hands, keyboard that sometimes can be used one-handedly should not be the same as a one hand keyboard.
Somehow, it should include all the uncompromising layout of a typical split keyboard, plus a set of keys on the left side, replicating the right side keys, but maybe with some adaptations. It could also be just another layer on the left side, but I don’t believe this would cover the need expressed by op.