I recommend calm and caution :)
You will quickly end up researching mechanical split keyboard market and start convincing yourself that 500-1000€ is not that expensive (sic!).
I made an adventure into mechanicals, around half a year on research and getting used to it. Configurability is amazing. Quality great. Noise was not a problem.
Ended up going back to some flat (laptop-like) dell keyboard and a vertical mouse instead. Because my hands started to stiffen up, hurt, the typical stuff. Keyboards were too high (with normal keys) and too expensive+weird (with low profile keys). I tried adding dedicated palm rests, changing habits a bit, changing bindings...
In the end I'm considering buying MS' Sculpt ergonomic keyboard again (it's the one that's actually split, low profile, with optional negative tilt. I think it's only sold with a mouse). It has a lot of flaws, but fatigue was not one of them.
What I'm trying to say: you may spend a lot of time, effort and money and still feel miserable. There are amazing builds out there, but don't forget non mechanical options exist.
RSI is a combination of "repetitive" and "strain". My take on it is that ergonomic keyboards that limit the amount of finger movement make your typing more repetitive - hence you need to be sure as hell that there is as little strain as possible.
I also went with a minimal split keyboard and quickly started to feel pain. When I however looked at my desk/chair/screen, I found that they don't promote the correct posture. Once these were corrected, I got no pain at all.
I think the right lesson is that this is a journey, rather than a problem you can throw some money at once and have it solved.
You described it well.
You can spend insane money on hardware and still sit incorrectly.
Funny you mention that, I was comparing Moonlander (https://www.zsa.io/moonlander), Glove80 (https://www.moergo.com/) and Cyboard (https://cyboard.digital/) ...
Thanks for the advice!
Moonlander ... I think I still have a printer template to check if it fits my desk layout well...
Hacking keyboard I like the most, but it's still a gamble and add-ons almost double my cost. Too much for companies that don't have any distributor locally...
Yeah I agree, the cost quickly rise!