Hi,
I've used 40% ergo keyboards for quite some time, but also disclaimer I'm not a professional expert.
I have a couple of points from my own experience:
- Smaller keyboards are great for ergo, I use a Corne atm, but:
1. The layout matters: many peolpe say that the shift to Colemak or Dvorak matters more than the keyboard itself. I use Colemak DH and I find it a lot more comfortable, but if can take a couple of weeks to get used to.
2. The keymap matters, how you reach for your modifiers etc, look into this keymap for ideas:
https://github.com/manna-harbour/qmk_firmware/blob/miryoku/users/manna-harbour_miryoku/miryoku.org it work for all keyboards, and you don't have to use all keys!
3. When you switch to a new keyboard it takes some time to get used to it, and the strain can be from that.
- The switches matter. In the beginning I jump on the train with everyone enjoying tactile switches, they sound and feel nice. But I get tired and fatigued if I use them for a long period of time. I write this on gateron clears, very light ~45g linear switches, which to me feel great. It took a week to get used to them, but now I cannot imagine typing on heavier switches.
- Wrist rests are not necessarily good.
- Tilting and such is not necessarily good either. This is individual. I like a flat keyboard, and I even think that for me low profile boards could be the best, then the keyboard is slick to the desk and you don't have any awkward tilting in your wrists.
- Take breaks.
- It takes time to converge to your optimal setup.
- General exercise is good. I can feel that I get more back strain etc when sitting at the computer if I don't exercise.
This was just some general advice, but maybe try colemak and a flatter profile?
And I should add that if the pain does not get better or even worse, it's probably a good idea to contact a specialist, some ppl get real long term issues from this.
Good luck!