Welp I don’t know why or what changed, but I no longer have any problem. I can not only turn my head 30 degrees left, I can walk the same distance away into another room before it cuts out, as I can with my X1 Carbon 5th.
I didn’t change anything, and the cables were connected right when I looked, but just for the sake of complete info, here’s everything I can say that has any possible bearing:
I’m running xubuntu 21.04 and the latest mainline kernel (5.14.9 for the last few days) (I actually maintain the mainline updater app forked from ukuu), and I rebooted several times as a side effect of looking at the cables and rearranging my desktop cables and stuff for a new laptop stand. Honestly I think it was the reboot. Just a random fluke software/hardware initialization glitch or something, the kind of thing that happens once in a while at any time on any machine. I never had any of the reported linux problems with bluetooth not working or resuming from warm start or the right-click thing. This version of xubuntu and the mainline kernels must both be just new enough to have all those things fixed. Everything just worked out of the box (though I haven’t tried to use the fingerprint or webcam yet)
I looked at the antenna cables. They were good, but I re-seated them anyway. The act of disconnecting them is part opf how I know they were good to begin with, because of how it feels when the come off. And when connecting, I connect cables like that by removing the card and connecting the cable to the card in the air between my fingers, so you can feel the cable connector snap into place flat parallel with the card with no guessing at all. Then just take a little care while moving the card into place. Plus you can rotate the cable a little to verify it swivels in-plane and is still snapped on. Also took some magnified pics of the connectors to see they weren’t mashed or something, and looked at the rest of the cables to see they weren’t pinched or gouged near the hinge or under the bezel. All good.
Although I re-seated the antenna connectors and now I have no problem, I also ate a ham sandwich and now I have no problem. I think it was actually just a fluke and the full power off reboot cleared it.