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.