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.
Thumb Shift makes a lot of sense to me. Most layouts switch layers using thumb keys. Conceptually, Shift is also a layer. I put it on the same thumb as my Symbol layer key and set up my symbol layer so that it doesn’t need Shift: All shifted symbols have dedicated keys.
Sometimes I need Shift with mouse actions. As I use the mouse right-handed, Shift has to be on the left thumb for that to work.