Hi Russ,
You could also possibly consider the Samyang 14 mm f/2.8 ED AS IF
UMC, which has remarkably good optical performance right out to the corners and very low coma, even compared with other main brands.
One caveat: some report their copies of this lens are not up to par, but I'm not sure if these concerns relate to the previous version of this lens, the Samyang 14 mm f/2.8 IF ED
MC Aspherical OR a sub-par version of this lens, the Samyang 14 mm f/2.8 ED AS IF
UMC. (noting the
UMC suffix indicates the more recent lens)
I suppose it's something of a risk to take, but having seen many of the coma results for ultra-wideangle lenses especially for more demanding full-frame use, it's one of the best irrespective of price point, even against the likes of Sigma, Canon and Nikon for low coma & low astigmatic aberration. Used they can be had for ~$3-400.
EDIT: Results for some Canon mount compatible ultrawide lenses now attached. Unfortunately they haven't tested the Samyang 20mm f/1.8 wideangle you are considering.
For more lens tip reviews see ...
https://www.lenstip.com/lenses_reviews.html
I nearly forgot: There is also the IRIX 15mm f/2.4....
https://www.lenstip.com/486.7-Lens_r...and_bokeh.html
Best
JA