Sorry for the wall of text, but here are some answers and motivations for some of the layout decisions, firmware choice, etc.
I use "physical layout" and "character layout", but have no idea what the proper terms are.
Tap and hold firmware can be tricky to get the timing right and it's sometimes too easy to press another key before releasing the previous, especially for space bar. It also introduces a delay in response for those keys which can be very bad when gaming, particularly games which require perfect timing. Jumping with space bar in a platform game, for example.
I chose not to use Workman, Norman, Colemak, Dvorak, Carpalx optimised, Programmer Dvorak, etc. for various reasons and settled on modified AdNW / BU-Teck.
1. Most of them are designed for standard horizontally staggered physical layout.
2. Some place characters in very weird spots.
3. I wanted a layout that has very little in common with QWERTY so I can keep my QWERTY skills alive while learning the new character layout on a new physical layout. The difference means I am learning a NEW set of skills for my new board instead of retraining the old (which requires UNLEARNING the old layout while at the same time learning the new, kind of hard after more than 20 years). It's working, too. I can still type just as fast on a "normal" board as I always have and I'm up to 20WPM on the new.
4. I prefer outward rolls to inward, but prefer alternation even more. Some layouts are designed specifically for inward rolls.
BU-Teck is designed for alternation, vertical stagger physical layout with thumb modifers, has well-placed characters for finger and hand use, is very unlike QWERTY and is well optimised and analysed for the most common trigrams in English. Unfortunately it also includes unnecessary characters and includes some weighting in its optimisation for German, which I don't need. I have thus modified it to suit me. Still may make some small changes, but in general I am happy with it. Happier than I would be with any of the other layouts I have come across so far.
I did also come up with my own character layout, but I haven't completely analysed it yet. So far it does very well in the online layout analyzer (http://patorjk.com/keyboard-layout-analyzer/#/main
), but I suspect it won't be as good as my current one for prolonged typing.
The number keys and characters have been very easy to learn so far. I find it's not so much a matter of symbols being associated with the corresponding number, but rather the position, so learning new positions for each independently hasn't been an issue. In fact it's a lot nicer to open and close brackets, etc AND type numbers with the layout. Could be partly because of the "numpad" type layout for the numbers and "strong" positions of the brackets (in the index and ring finger home key positions).
I do use the arrows and nav keys, but not as often as I thought I would. I find them quite okay to use, since I change my hand position a little when editing. I use shift in combination with them for selecting areas to copy, paste, etc. For the 40 key version I would drop the navigation cluster and arrows.
The Esc and Del buttons are well within reach of my pinkies. About the same as Q and P on a "normal" board, but I would certainly consider putting them on thumb buttons if I had more. Sometimes it's nice to have ESC in the default "panic" location, top left, but Del has no particularly strong reason for being where it is.
The 40 key layout isn't finalised, but it would probably have something like the same alpha and symbol layouts as the current one. For the function layer it would have:
F1 on Esc, F2 to F11 on the top row, F12 on Del.
Numbers on right hand, 1 to 5 in middle row, 6 to 0 in lower.
PgUp, Home, Up, End on left hand home keys.
PgDn, Left, Down, Right on lower row.
That way I can do all the edit functions with the left hand while keeping my right on my mouse.
There really are a lot of options, perhaps the best (if I were to make these available as a kit for example) would be a board with the same physical layout as the current one, but with 2 more thumb buttons per side and completely reprogrammable (would allow more layers and possibly some macro keys), although I must say that I am very happy with the current design and "only" 4 thumb buttons per side as they really do everything I need and are very easily reached with as little movement as possible.
I will experiment with 6 thumb buttons on the acrylic version. Doesn't hurt to have extra buttons, I don't have to use them all