Your roads sound like a dream compared to mine.. if they really are as hard pack and devoid of serious gravel chunks then you are definitely on the right track with file tread type tires. My experience with gravel comes with my daily commute, right out of my door I have 2 miles on some marble sized chunky and wash boarded gravel coming off my hill, I drop about 400' vertical in those two miles. It's down right scary right now with the marble sized gravel on top of hard pack dirt right now. However, earlier in the year the road was mostly as your describe yours, hard pack dirt and only a little bit of loose in the corners.
I don't have any direct personal input on any of those tires.. but I have used two sets of Donnelly tires similar to the MSOs.I currently have the Strada USH 32's on my bike and used to have the X'plor USH 35s for my commute. If you are on smooth gravel as you are describing the Strada USH would be fantastic, they are fast and feel good tubeless, If my bike would accept them I would have gone with the 40mm version for extra confidence down this hill. The X'plor USH was pretty meh for me, it only came in a clincher and they feel less confidant than the Strada USH 32s I now use. I expect the MSO 40mm will perform quite well with the small knobs to grip your hard pack and some loose stuff but won't be as fast as the Strada USH or X'plor USH if that's what you value.
My local shop has been stocking Teravail Cannonball 38's with the durability casing that the local adventure riders have really liked so far this year. these are folks riding the Denali highway, the Denali park road, and various other gravel climbs that have similar to worse composition to what you are describing your roads as. They have a nice file type tread in the center and some pretty beefy and long lugs on the outside for cornering in loose. These were going to be my next set of tires, but I got a good deal on the Strada USH instead. The Cannonball tread is very similar to the Boken tires you linked but perhaps a touch more aggressive.
My dad's CX bike came with the 38mm WTB Riddlers, they are nice on the short "parking lot" rides I've done through his wooded yard. Feel like they roll fast and feel quite supple. I was riding over dirt and off cambers roots with confidence. Again as you probably noticed, very similar tread to your other choices.
If I was in your situation.. and my bike fit 40mm tires, I would be using the 40mm Strada USH despite the seemingly lack of tread because the 32's have been working very well for me in rougher conditions but still roll fast.
