Then there's the 40mm pancake. Probably not quite wide enough for you, then there's the electronic STM that likes to quit every now and then. But for $150, not bad.
I love my Shorty Forty! Not really wide, of course, but a very natural field of view, for my eyes, on a 5D.
I wish I had kept my 28mm 2.8! I acquired it and a 24mm as pre-owned lenses, from the same seller, and traded the 28mm without trying it much, and only on cropped frame. I was then disappointed, over time, with the 24mm, and quite pleased with my EF-S 10-22mm, so I traded the 24mm. I fear I let a extra-sharp example of the 28mm slip through my fingers. (Of course, the 10-22mm EF-S cannot be used on a 5D.)
The only really wide lenses I have at this moment, that I can use on my 5D, are Nikon-fit, coupled with a Novoflex EOSNIK adapter; manual focus is necessary, of course. The Vivitar-branded Tokina-made 17mm f/3.5 can be fun to use, though it is certainly not up to an L lens in performance, but at less than $150 US, a decent value. My Nikkor 16mm Fisheye 2.8D is, well, a Fisheye, a specialized lens. Notably, it does not quite focus to infinity when used on an adapter, which is not unknown when using Nikkors with CRC ( close-range correction ) with an adapter. I own some wonderful Nikon cameras, so that is why I try Nikon-fit lenses on my Canons. I am not recommending (for or against) acquiring Nikon-fit glass to use solely on Canons.
I would like to try an EF 20mm 2.8 on the 5D. It has not received many favorable reviews, but if the oft-cynical Ken Rockwell is correct, that is because it is indeed horrible on cropped-frame cameras, especially when shooting a flat subject that is parallel to the sensor, while being much better on full-
frame cameras, shooting a three-dimensional, real-world scene. This lens is about as wide as it gets, before one has to start spending significant money. I dream of the $2000+ EF 14mm L!
An alternative, about which I would like to learn more, is the Tokina 11-16mm 2.8, which supposedly works at 16mm on a full-frame camera, with little or no vignetting. This lens is made for cropped-frame DSLRs, but unlike Canon-made EF-S lenses, will not be struck by the mirrors of full-frame Canons, if at least one internet review is correct. (I really do need to find that review again, and make sure he was referring to the Canon-fit version, and not the Nikon-fit version, with an adaptor.)