BillH: Old Man Mountain was bought out by The Robert Axle Project. Not sure of your rear axle diameter, but I see they list a 12mm rear axle in various lengths; from 172mm long, 167mm and 160mm. These axles are made to fit their racks. Link:
https://oldmanmountain.com/product/...mm-thread-pitch-and-167-160-or-154-mm-length/
Smorgasboard: I too would question attaching that seat post mounted rack on a dropper post. For one thing, you need to reef down on the attaching bolts pretty good to prevent the rack from fish tailing on you.
Before I settled on the front and rear Old Man Mountain Racks, I went for a good long time running a front OMM rack to fit my side panniers and a rear seat post rack and bag.
I settled on the Portland Design Works Bindle Rack with the Revelate Designs Terrapin dry bag, which is designed to fit and work with the PDW rack. It worked just fine, then again, I don't have a dropper seat post as you do. But the main thing is, you are really constrained on what you can bring along. On a warm day going long distance, I like to carry extra water, my ultralite seat to rest up in as well as the standard air pump, tire patching kit and spare fat bike tire tube. And that back is pushing the limits on the amount of stuff you can carry. For your purposes, it sounds like it would work.
But beware: Reaching full tire shock compression, my back tire would hit the bag where it fits on the seat post in the below picture. Your ideas of putting your rack even lower then what I have will definitely result in the rear tire hitting your rack.