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 tried using shift on my thumb as the hold action for space, but I was not a fan. The problem for me actually things like the end of sentences. I tend to still double space between sentences, so this meant every sentence was tap-tap-hold on the same thumb for space-space-capital. Even worse, I was testing a layout where you did exclamation point with that same thumb by chording the layer button and shift, so such a sentence would have 4 sequential uses of the same thumb!
Now, I actually keep a dedicated pinkie shift key, but the other modifiers are on the thumb. My thumb keys are space (live alone) and all things I rarely roll though (delete, backspace, enter, tab, escape), so on all the ones you don’t roll through, I’ve put the modifiers without any delay: The moment you press them the modifier activates, and then on release it deactivated, then sends the key tap if less than the tap time. This lets you go really fast without misfires.
One thing to remember: in English, the space character is about twice as common as the letter e. It is, by far, the most commonly pressed key, so you probably shouldn’t overload it!