Swapped out all of the SMA parts, now it's SMPM on the DUT and all 2.92 on the scope side (properly torqued). Reflection didn't go away.
So it's probably the PCB, and the via transition is my lead suspect. Not enough to impair functionality (or even close to it), but something I can't unsee now.
At some point I'll compare TX1 (shown here), TX0 (left via in the above screenshot), and SYNC (TX-only channel with different layout that has return vias).
If SYNC doesn't have this artifact and TX0 does, I'll say case closed. If all three have it, then I'm more inclined to blame the SMPM launch.