I'm not exactly the voice of experience you're looking for, although I've built a couple of lap dulcies and have "fooled with them" a bit.

I can think of at least a half dozen "casual friends" who were known for always showing up with a different instrument every time you saw them, who eventually settled on the mountain dulcimer and became reasonably decent players. We'd whisper behind their backs that it was 'cause they were too lazy to learn any of the other instruments they tried so they picked the one that looked easy.

The dulcimer is fairly easy to play, for most people, - up to a point, but takes at least as much study and practice as any other instrument to advance beyond the basics.

With a guitar, although the wrist may be in an awkward twist, the pressure to fret the strings comes mostly from the grip within the hand. With a dulcimer, as commonly fretted, the whole arm/wrist/finger has to push down on the strings. Whether that may require different muscles more or less suited to your condition is something you'll probably have to try out to know. There likely would be a bit more "movin' around" of the whole forearm and wrist than for a guitar, which might make it more user-friendly in your case.

Since dulcimers are usually only fretted for diatonic scales, learning the "modalities" is generally part of learning the instrument, and for "less than well-advanced" players, the absence of all those in-between frets may dictate that you play "different kinds" of music than what you've learned on the guitar - until you get past the stage most of the above cited friends ever reached. The full range of music certainly is within reach of the instrument; but few of the campground players I've known have gotten much past "Irish." (Not intended as a denigration of the kind, but it does tend to a more diatonic/modal structure than rockabilly.)

The "other alternative" that a few friends have gone with when the wrists went were lap steels and/or dobro, using a slide from the top to keep the kinks out of the wrist. I can't say that I really prefer one choice over the other, dulcie or steel, as either can be good instruments, and either can be instruments of torture if you don't work with the one you pick.

John