Thanks for pointing out the Bluetooth driver update; I updated it via Windows optional updates.
Unfortunately, the stuttering remains after the update, with or without other Bluetooth devices connected.
Some other observations from my testing in the Australia and Britain time trials:
The stuttering is the worst at the beginning of the session, improving after a lap or two, but never completely going away.
Turning the controller off and on again improves the stuttering, but does not completely fix it.
The stuttering is not exactly a decreased framerate, although the framerate detected by Steam does decrease. It's more like several frames at the normal framerate, a pause with no frames, and then several more frames at the normal framerate.
No other games (including Forza Motorsport 7 and Forza Horizon 4) give me these issues, which is why I'm on this forum 🙂
I will likely give up on wireless for this game anyway. I just did a reaction time test, and it clearly showed an extra 50ms input lag for wireless over wired. I do think 50ms matters in this game as the rhythm of these circuits is very tight.