Still not used to splits after a year…how do you type one-handed?
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.
There are a few different options, though none of them are without trade-offs.
You could try getting a pointing device integrated into your keyboard, like a trackpad, trackpoint, or trackball that could be used with a single finger while the rest of the fingers are more or less still in the home position. For example, a tractyl.
You configure your left hand side (assuming you mouse with the right) to have extra layers which give access to a full keyboard from just that hand. The first thing I would do here is figure out how much you really need to access... Do you just need copy/paste/cut? Do you just need alpha keys? Do you really need a full keyboard including symbols and alt, F-keys, etc? For an extreme version of this, this keyboard using a Taipo layout gives either hand access to a FULL keyboard with only 11 keys per hand, and you have almost 3 times as many keys to work with. You can type entirely with just the left side or entirely with just the right side with that keyboard, though going that small will have a steep learning curve.
Get something with even more keys than a sofle, but for just the left split, and you'd only use those extra keys while one handed typing, so each one of those keys is a duplicate function that can be achieved in a different way when you're using both hands. You could even use a 60% keyboard as your left split, as it would mostly just invade the space between the keyboards which you might have free due to the split.
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.