hi graham,
the files I think you will find will be too large for Registax.
DSS will also struggle as it doesn't like images of varying sizes.
I'd recommend RegiStar, I've used it to combine nb and rgb data from my dslr - (the NB data image is 1/4 the scale of the regular dslr image after using superpixel mode).
https://aurigaimaging.com/
You do need to convert CR2 to tiff first and it can handle fits. But it is so quick and easy to use.
Cheers
Rusty