Hi Carl,
The Sigma 40mm f1.4 Art lens is probably the best as a mid-wide, the 50mm f1.4 Art and 35mm f1.4 Art lenses are also excellent. SOme Canon astro shooters love the Nikon 14-24mm f2.8 with a Nikon/Canon adaptor (Tony Hallas believe it or not). THere is only one Canon lens that comes close to the Sigma 40mm f1.4 on paper, it is the 35mm f1.4 L II USM but it cost a motza, more than double the Sigma. I have also found the Samyang 14mm f2.8 UMC to be excellent and on par-ish with the Nikon 14-24 for low coma. Have a good look at the lens tip website for reviews with optical tests.
If 40mm is wide enough - the SIgma is a killer.
If you want just one lens for a Canon FF, that should be it, next choice perhaps the sigma 35mm f/1.4 Art
Of course that advice holds for A Canon EOS FF mount, getting these lenses in the Canon RF mount - one would need to check availability TBA ??? check for further updates .....
https://petapixel.com/2020/05/12/sig...lenses-report/
or less elegantly use an adaptor (check with Canon) .....
https://www.canon-europe.com/mount-adapters-ef-eos-r/
For interest here is a related discussion I started on the same issue a while ago.....
http://www.iceinspace.com.au/forum/s...d.php?t=177331
Best
JA