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 use thumb keys with mod-tap (control and shift, combined with some other commonly used keys such as space and enter), so far it works great.
And in case you don’t know, for QMK, there is an option https://github.com/qmk/qmk_firmware/blob/master/docs/tap_hold.md#hold-on-other-key-press that allows faster hold when using mod-tap.