It does seem amazing to me how hard it can be to find decent mittens. I lost one fleece mitten early on the AT this past year and was looking in outfitters all along the trail and found that no one sold mittens, I think with one exception of a fairly heavy, bulky ski type mitten. I think that outfitters sell gloves because people just far prefer gloves over mittens. I.e., maybe it's an education issue.
It's a no-brainer to me to have a thin glove "base" layer, a wool or synthetic mitten mid layer, and a light shell. I'm not to0 particular about the thin glove; I've never found one that's durable, and they're all too expensive. If someone has a better idea, I'd love to hear it as it's expensive running through these things over the years, the finger tips seems to shred and get holes way too fast.
For mid-layer I just bought a pair of Fox River double ragg mitts, "extra heavy weight". These are heavy --- 5.7 oz (163g) in size large. The size large are slightly long on me, slightly tight in the wrist and thumb, so not an optimal fit, but okay; I've got them stretched over two stacks of tuna fish cans to try to stretch out the wrists.
The "double" part of the "double ragg mitt" name is correct, there are two layers. The outer is a mostly wool shell and the liner is more acrylic than anything else. The liner is stitched to the shell at the edge of the wrist only, i.e., you can pull the liner out of the shell (good to help dry faster, at any rate). Or of course a person could cut out the liner and stick with the shell, something that I might consider to reduce the weight and make the fit more comfortable. The shell is 85% wool and 15% nylon, while the liner is 67% acrylic, 23% wool, and 10% nylon.
For a rain shell, I've found the MLD eVent rain mitts to be more durable than I had feared they would be (even for a trekking pole user). But overall I haven't had to deal with all that much rain in recent years, and much of that was warm enough that I didn't use the shells. Hopefully they'll hold up to one more thru-hike!