Is there a way to get the Canon DSLR driver to trigger capture without downloading the image?  It can be set to save to the camera's internal storage, but I can't see a way to get it to not also download the frame.

With the Canon R5 download is taking me about 2.6 seconds over USB3, so with 5 second frames I lose more than 30% of light gathering time. I've been having the camera capture on its own by holding the shutter release via a cable, but when I tried to have the driver handle it I got far fewer frames even with dithering off.

Yes it's possible with recent INDI. You can set it in the GPhoto driver to Ignore the image so it won't be downloaded.

I got rained out tonight but it worked perfect in a dry run earlier in the day-- 6 10s exposures in 63 seconds, totally reasonable. Thanks!

I tried this and worked well. I had one issue is when a meridian flip was ready, the camera driver could not stop taking images and prevented the meridian flip. Once I stopped the capture it then carried out the flip. I had no indication that this was fixed or not.

Maybe test it just on the verge of a MF and see if it works.