Personally, I don't consider rotary tools all that useful for this hobby. Perhaps in terrain construction, if using harder materials, or rough modification of large kits, but the rotation speed (even on the lowest setting of most adjustable tools) is generally too high and proper control is difficult, compared to hand tools. I have two (beefy B&D RTX and a tiny, high speed/low torque job from
HF) and they pretty rarely see any use for modeling, especially not for fine tasks like mold line removal. I'm currently mangling a large resin cake topper for resculpting, though, and the RTX is proving invaluable. For my general modeling, though, not so much. I even opt for my pin vice when drilling over the cordless drill/driver, let alone the much higher speed rotary tools.
Since you have a health issue, though, it's well worth considering your options. Have you tried modifying your hand tools for greater comfort/less strain? If your files are too fiddly, consider beefing up the handles. Naked tangs (with a traditional taper, at least) can be driven into undersized holes in dowel sections to give you chunkier grips (I chopped up an old wooden broom handle for some of my larger metal-working files, but empty handles are available if you prefer a more professional finish). Most model-sized files, though, have narrow rods for handles, instead of traditional tangs (with or without rubber covers, which can always be removed). Those handles can easily be mounted into a large pin vice or razor saw handle (X-acto style saw handles double as large knife/chisel blade handles - just check that the collet will fit). A larger grip will relieve a lot of strain on your hand, like mounting models onto corks or paint pots when painting.
If hand tool use is still too strenuous, even with more comfortable grips, consider getting a flex shaft over a Dremel or similar. Not the accessory (although that will reduce hand fatigue and increase control if using a rotary tool) - the actual tool, as used by jewelers and wood carvers. Same idea as a suspended rotary tool plus flex shaft, but the dedicated tools generally have higher torque and, more importantly, can function at much lower RPM. The foot pedal speed control is quite nice, as well. They can run you hundreds of dollars, but there are less expensive versions available, on par with mid-range Dremel kits.
Harbor Freight's offering, for example, only costs as much as an entry-level Dremel (but may, like many
HF power tools, require a little TLC before use to really shine - read the reviews, if interested).