TL/DR: Almost anything repetitious can be programmed to execute with 2 key presses. If you can press Shift to capitalize a letter then you can press Caps Lock to access another layer. User layers and improve productivity!
I've lurked this topic but want to say that QMK and Layers are the reason I have no need for a ≥75, especially since I don't do enough spreadsheet work to warrant a num pad. If I did then I might add a 10-key rather than use a full size. Or I might program a 10-key. The key is learning
how to work with a small keyboard. Read on...
In Layer 0 my first change is to map MO(1), or Fn or whatever, to Caps Lock. This puts all F keys only a pinkie away so I'm good TKL. I use layers only on a momentary basis so I don't do a full toggle, just momentary. Access to Layer 1 involves simultaneously pressing MO(1), which is mapped to Caps Lock, plus one other. This puts my left pinkie to work, but it's incredibly natural at this point. One other important change on Layer 0: since I'm a Mac user, I switch Alt and GUI to mimic the Apple layout.
Layer 1 helps with work flow. If using a 65, arrow keys are on the right but if not then map to IJKL (which I prefer to WASD) with U for PgUp, O for PgDn, N for End, and H for Home. Easy to put media keys in R2 so map Q for Stop, W for Play, E for Rewind, and R for Forward. I like to take screen Caps during video conferences when someone is presenting, so map that to C (otherwise its a 3 key combo on a mac: Shift+Command+3). I use Z and X for common email addresses, which makes logging in much faster when I don't use Google. Oh yeah, map P for my phone number.
I've considered remapping arrow keys to WASD and creating a 10-key (9 actually) using UIO/JKL/M<> but this would require more changes to my muscle memory than I'm willing to make right now. But at some point if I start to think I need a 10-key again then I'll try this first.
Layer 2 gets common email language using mnemonics to prompt by letter (e.g., Intro to I, Closing to C, etc.) and
macros for common routines assigned to numbers. Some of these macros are software specific, so think about what might help you with software you use regularly. With QMK it's possible to program almost anything you type, so think about how many times you type certain keystroke sequences. Then poke around for the commands if you're not familiar. Almost forgot, to access Layer 2 you can map MO(2) to right GUI (this is default but since I'm a mac user I already switched GUI and Alt in Layer 0 so I'm actually using RAlt, if that makes sense). If you don't use Layer 2 you might map a redundant MO(1) to this key.
At this point, more keys on my keyboard only slow me down. Even with a 65, having PgUp and PgDn way over on the right is more inconvenient than mapping to U and O (get it? U for Up and O is the Other way). I'm excited, however, to
try a TKL FRL like the Eclipse. I'm interested in mapping additional layers with more macros, so I am thinking about using the top 3 keys above the arrows to toggle these layers. But there's a lot to be said for staying on the home row with a 60% or 65% layout once you customize it and get used to it (which does take a little time).
That's it. These are all really simple combinations. If you type the same sequence of 3 or more keys multiple times a day then you might consider using Layers to create a macro. And you don't need to use Combinations; you could
Tap Dance -- use a double tap (or triple tap) to initiate a specific key result. And I'm exploring the
mod_roll function. Last, there are some extreme limits on programming (dynamic macros are limited to 128 keypresses) otherwise the only limit is your imagination.
If anyone has some useful shortcuts they really like, please share!