I've only had a keyboard capable of layers for one week. It's a moonlander, so I still have access to the number row.

My experience is that my symbols / numpad layer is a big win for the symbols (especially parens, ~, +, and | ), but the numpad is only faster if I'm typing multiple digits. For me, the stretch to the number row for a single unmodified press is quicker than a chord of easy keys or consecutive presses of easy keys (as done with sticky OSMs). That said, I feel that I have mediocre coordination but reasonably long index, middle, and ring fingers.

I agree with most of your points. See the BigBag links page, where there's a link to an article on chording vs sequencing by XahLee, and also the Extend page where I have a section on sequencing.

On a side note, I'm one of those strange people who believe you can go too far in reducing board size! Lots of typing enthusiasts seem to want a 34-key board these days but I don't agree with that for my purposes. Around 6×4×2=48 keys plus thumb keys is what I'd probably like best. A 34-key board is probably good enough for typing simple English text and a few symbols, but once you want to do anything more fancy I believe in having both layers and some more keys that you can reach with short stretches. The trick is to keep the often-used stuff near the home position, and still have those extra keys for more rarely used stuff. So I don't mind a few short jumps as long as they're rare and make it easier to remember where I put stuff!

As an example, I use the number keys to release glyphs from several of my dead key layers. If I am to enter a lot of numbers I prefer to use a NumPad layer (my Extend2 layer) instead of reaching up to a number row, but I still want the number row! Both for the occasional single or so digit entry where accessing a layer is no real benefit, and for mapping stuff to the number keys on other layers. If the number keys themselves were only found in a layer, releasing glyphs with them would mean more of a tap-dance.

I realize that with a programmable board you can introduce key chording concepts that allow extremely powerful layering from the home row, but unless you can do something like that I don't quite believe in taking size reduction all that far. I do like the idea of at least three thumb keys on each thumb though.

You have to realize that there are several kinds of layers! Or rather, ways of accessing them. Chording layers which the usual switch-type modifiers (Shift, Ctrl, normal Extend etc) provide are great for navigation and things you want to stay in one layer for a while to do – as long as you don't end up playing "keyboard Twister". Typing many key strokes in the same layer while holding down a modifier gets tiresome and gymnastic, which is why we got the CapsLock which is a lock-type modifier.

I use a sticky shift with a timer on it, which is a special kind of latch-type modifier. This helps avoid some twisting, and feels great.

For one-shot output such as special characters, PowerStrings or command shortcuts, I greatly prefer either latch-type modifiers (you press the modifier and let go of it, then a release key to produce the result) or sequences. Which one it is depends on what I'm doing. Latching for things I use often and that may be logically placed in layers – for example, accented letters – sequencing for things that are used less often and may be hard to remember in layers – for example, I use the Compose key to produce the below with the sequences 'thoth' and 'saybig':

( Θώθ)  ～(  See "DreymaR's Big Bag Of Keyboard Tricks"  )
  https://dreymar.colemak.org

I agree there is no point to just try to reduce to as few keys possible - there is a happy medium between fewer keys and less movement but more key presses vs more keys and more movement but fewer key presses. But all things considered, I think that happy point *is* at relatively small number of keys - probably around 36.

If you take the main body of they keyboard - that's 3 keys for each finger, except for the index fingers which have 6 - that's 30 keys. Then, I'd consider the optimal number of thumb keys to be 4 or 6, leading to 34 or 36 keys in total. That's the greatest number of keys you can easily type without moving fingers more than one key unit. I'm sympathetic to the idea you might want an extra outer row for pinkies in which case that gives you an extra six, leading to 40 or 42 keys. Any more than that, you have to start moving your fingers quite large distances, which more than negates an extra key press - particularly when such extra key presses are comfortable thumb keys.

A lot of the popular smaller keyboards and layer systems seem to based around the 36 key mark, so I can't be the only one thinking that must be close to the sweet spot!

Yeah, there is definitely a mental load issue. The fastest Colemak typists both seem to avoid going for too fancy solutions as the main load of typing at high speeds isn't physical but mental. Complexity comes at a cost.

I'm currently trying to use a trick to type punctuation followed by a space and, if relevant, capitalization. In theory, that's quite the gain as I'll save my fingers from having to curl down to the lower row for punctuation and also get away with pressing the Compose key plus a home row key to get 2–3 key presses – also saving the hassle of reaching for the Shift key! But in practice my brain is too daft to get any real benefit from this brilliant idea so far, so if anything I'm slower rather than faster when using this trick. It could simply be about getting used to it, of course, but I've been trying it for a while now and I'm still confused...  ლ(ಠ益ಠ ლ)  It is comfy though, which is nice.

Today I started using a thumb Compose key for this instead of an index one, maybe that'll be better. But at any rate, I'm definitely feeling the hassle of fancy layering these days.