I haven't gotten down to the 40s, but my daily driver is a 62-key Dactyl Manuform Mini. Admittedly, 3 of those keys are sound driven, so I could see getting down to 58 pretty easily, but after that, I'm not sure what I'd ditch.

The one thing that never really worked in my brain was having multiple layers and a dedicated layer up/down key. I tend to store keys on things where I'd normally hold instead. For instance, my shift keys also hold ( and ) when I tap instead of hold. Same with control and alt. The only layer that I really use would be similar to that raise key, to turn my top row into F keys.

I really wish there was a somewhat affordable way to get good legends on keys that represent this information. I still have to take a beat to remember what | is on my keyboard every time I need it.

I'd highly recommend at least giving a smaller keyboard a go though. The kits that I've seen are fantastic if you want some hands-on tinkering, but also the prebuilt stuff is slick too. I had zero soldering experience going into it, and now it's turned into not just learning about how to use a soldering iron, but also 3D printing, as I wanted to test out a bunch of different keyboard shapes and that was a low-cost way of experimenting.