Thanks for your observations. You are not alone!
This lines up with my experience very closely.
(Were the brushed 15 Pro colors like this? As-in this variable? Or, is this the first year to be wary about grip across different colors?)
I also go case-less. I was able to test the black Pro and white Pro at home, and extensively tested the other colors in-store (on 3 separate visits!).
I initially got (and still have) the black one. I almost returned it because, while being the most tacky, I did/do not like the overall feeling.
I was going to go white, because I think it looks great. But that thing is slippery as a fish. I just don't trust the grip.
After reading your commentary I now have some FOMO about the Natural Titanium.
In-store, the Desert was pretty close to the white, though perhaps having slightly more grip.
In my experience the Natural was closer to the black in terms of grip, but the "squeakiness" was less pronounced.
I could see how that could potentially make the Natural the best middle-ground choice between feeling good in-hand, yet providing enough grip.
I ended up keeping the black Pro, and am out of the return period at this point.
It's almost as though the "bar of soap" white Pro is the most "natural metal" (despite there being another color called "Natural"), while the other colors have various levels of PVD(?) coatings that are providing these grip discrepancies.
I had a natural 15PM last year and didn’t really notice a difference in texture since I wasn’t really aware and was thrilled with the grippiness of the natural finish. Now that I settled on the black, it’s easily the most tacky/least slippery case free iPhone I’ve used in recent years, which is great, but I have no idea if the 15PM had varying textures across the board