I'm not trying to drop the stem all the way down to the head tube (what I would consider "slammed"). I'm trying to lower it about 10mm from its current position, so it will have around 20mm of stack between the head tube and clamp, instead of 30-35mm as shown. I want to do this because I got a fitting on my other bike (which is also a CX bike) and the fitter recommended a lower stem height, and I'd like the fit on the two bikes to match since I use this one for indoor training/winter riding and the other bike the rest of the year. My other bike has a different shaped headset top that provided more room underneath for adjustment (plus no brake hanger).

This bike has an alloy steerer tube, and a "star nut". I've never cut a steer tube, but I'm fairly confident I can pull the star nut out, cut the tube, and re-install. I suppose I could bring the fork into a shop and ask them to cut it for me.

I think a shorter conical spacer seems like the only way to lower the stem, but I'm open to other suggestions if you think that won't work? I'm not sure what you mean by "space the bearings".

I ordered the spacer that the other poster linked above. The one I bought is listed as 10mm height, with a 46mm outside diameter at the bottom. This seems to match the size of the conical spacer I'm replacing, it's just a 10mm height vs 20mm. This seems like a pretty simple job, but maybe I'm missing something? It's really not that big of a deal if the stem is slightly higher on this bike than the other so if it seems like a can of worms I'm opening, I'll just leave it as-is. (I still need to cut the stem regardless).