I’m thinking of switching my thumb keys (currently space and enter and two of them go to a symbols layer) to shift. I find that shift makes me anxious when typing because i have to partially leave the home row, and as a result my pinky placement when typing is bad, my ring finger tends to take over and my pinky has become a mod presser and not a typing finger.
I have tried to overload the home row keys as modifiers but that is a huge mistake as it causes misfires or takes TOO LONG to aviate, there is no middle ground with that one.
I am currently using thumb shift on all my keyboards as it feels very natural: no need to move the whole hand (unlike classical shift) and no priority conflicts (unlike HRM shift: with HRM, if hold is preferred then you shift letters by mistake, if tap is preferred you end up typing erroneous extra chars instead of shifting).
Actually I combine enter and shift on the same key, giving priority to hold/shift. Since I don’t need to roll enter, there is no conflict.