Hi Ian,
It's been a while but I'm still looking at that Speedmax and actually have the money now, the CF 8 disc mechanical ultegra. I've done a bit more time on my current TT bike and I think the fit is pretty good, a lot lower than the Shiv but still comfortable and I've managed to get some closer measurements.

Edit: I just watched your video on how to do the measurements, so I've re-done it.

My height - 182cm
My inseam - 90cm
Stack - 670mm
Reach - 390mm
Saddle height - 805mm (I measured at the same angle as the seat tube, however, I think I sit a fair bit forward of that point. My current bike has 170mm cranks.)

The canyon site recommends size M, and my measurements do fit(Edit: well they did when I measured to the centre of the pads, but not measuring to the back). However, I'm conscious that the speedmax has a significantly steeper seat tube angle(80.5 vs 75 on my current bike), which will bring the saddle further forward, especially given I have it quite high.

My other thought is that with the medium the saddle will be closer to the top and I'll need a lot of spacers for the pads. I could potentially swap the 90mm stem out for the 80mm or 70mm to shorten the reach(I was told I could do that when I talked to a canyon rep when I was previously looking at this so I assumed it's the same).

I do currently have similar profile design pads on my current bike and have tilted them back which does shorten the reach a fair bit(see photo)

I don't have a problem with the tall spacers if the medium is the way to go, I'd just appreciate your thoughts.

Edit: After re-doing the measurements as instructed in your video, it's now looking like the Medium is my only choice and I may need to swap the stem out.