Helping a friend: if one pushes/pulls on the side of the rim, there is noticeable but very slight movement. One can feel it, and also see the brake rotor move (but not enough for the rotor to hit the pads). Every component seems to be in order: bearings are new and smooth; freehub body, cassette (Ekar, if it matters), and end caps are properly installed; the two axle o-rings are in place and intact; cassette lock ring and thru axle are tightened to spec.
Mechanic flagged this as a factor in adjusting the shifting, and was looking for a schematic. (We do have a Spinergy video showing disassembly.) As best I‘ve been able to tell, it does not affect the shifting & can be ignored. But I figured I’d check here.