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.
I use miryoku and typing uppercase is annoying. You are either swapping shift keys as you type or you have to use caps lock (ewww). Let me know if there is a secret.
Another alternative to try would be one-shot-shift. Instead of holding the shift key while typing the letter you want to shift, you would tap the shift key once and then tap the key you want to shift.
After that one key press the keyboard goes back to typing unshifted characters.
I haven’t been able to try this, because my keyboard doesn’t have the 6th column with the shift key.
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.
Did you try bilateral combinations? They reduced accidental misfires to almost zero for me:
I use it for shift, and I like it so much more, because pinky would be overworked otherwise. However I use Flactyl and it has a ton of thumb keys, so your milage may vary if you do not have enough keys.
One of my thumb keys is Shift, and the opposite is Nav (arrows on one hand, mouse on the opposite). Last time I tried home row mods, a couple months ago, I caught up with what somebody said was the settings you really need for home row mods, and the misfiring problem I had before was much improved. But my fingers felt overworked, my thumbs felt underworked, and switching modifier keys when I needed to type a letter key on the other hand was work my brain isn't used to. For the same reason, I have not put 'e' on one of my thumbs as in the Maltron layout: thumbs for mods and space, fingers for letters.
Enter is a key I don't type nearly as often as space and backspace, so Enter goes on my little finger and the latter two are on my thumbs.
While I'm holding forth, homerow mods are certainly a thing on Colemak-DH (see Miryoku), but I think the better your home row is for typing letters, the worse it is for homerow mods. Letter/mod combos should go on less common letters.
On my Dactyl Manuform I have right thumb managing enter and space and left thumb managing shift and backspace. I also use the option to hold for shift with tap being tab and backspace is Ctrl if held.
I'm not using the thumb cluster for shift and other mods, but rather homerow mods. It takes a bit to get used to, but I feel like it is very confortable. I have S/shift and L/shift (tap/hold), so if I need to shift a key from the left hand I use the opposed hand to shift and vice versa. Also pressing both S and L at the same time will toggle caps lock which I find very convenient.
Homerow mods allow me to keep my fingers on/near the homerow and reduce A LOT hand movements.
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.
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!
I just have the innermost thumbs set so it's space and enter (left, right) when tapped, but the right is shift when held. Has been pretty easy to get used to and have been using for years at this point. Can recommend.