Wellll, I have arthritic thumbs, and have had considerable difficulty fitting 650B x 48 mm Panaracer Gravelkings to the rear wheel (a Ryde Rival) my Mercury.
I did switch from a tubeless rim to a DT Swiss rim (recommended by Thorn). Andrew at Tall Tree Cycles, my LBS, did a nice job of rebuilding the rear wheel.
And, I discovered the Kool-Stop Tire Bead Jack, suggested to me by Danneaux, and -- voilà! It works very well indeed. Not expensive -- Cdn $27 from an LBS.
I'd be remiss not to mention that, dealing with the last 8 - 10" of the bead, I found it helpful to use a 5" QR rubber tie to hold the bead in place after I'd eased it over the rim with the Kool-stop jack.
And, on the matter of arthritic thumbs: My GP prescribed a cream with 10% diclofenac. I use it esp on my left thumb, which has an osteophyte-in-formation. Diclofenac is the active ingredient in Voltaren -- but only at 1%. Face-à-face with arthritis, 1% is, er, inadequate.