Upload & Sell: Off
Well, I don't fundamentally disagree with you on any material point, its just struck me as unfair apparently singling out the 24L for criticism concerning coma when all fast wide lenses sadly have this defect till well stopped down. Even Zeiss at the same apertures I strongly suspect.
In other words I felt it would have been fairer if you had said "don't get a fast wide lens for astronomy" rather than "don't get the 24L". Its the class of lenses that is the issue, not Canon's version of it. These lenses are not optimized for astronomy and must balance many competing optical pressures, its a miracle frankly that any of them do so well as they do, albeit at a massive price point!
Some photographers will go frothy at the mouth with any stellar coma in the corners. I wouldn't advise them to buy a fast wide at all, and to never shoot wider than f4 or f5.6 if they are that critical. Its these people who should use driven mounts (although aurora photography might well demand the short exposure times the fast wides permit).
Whether the coma of a particular lens is acceptable to a particular photographer is subjective, and is why the OP should rent, and try, before buying. I've seen lots of good night sky work on FM taken with a Canon 16-35 at f2.8, and the 24L would certainly totally smoke such a zoom at any wide aperture.
Incidentally I've often wondered at what focusing distance are the Lenstip coma tests done? Some lenses will be optimized for coma correction at infinity, some for closer distances, and unless the tests are always at infinity I wonder what they mean for astronomy?
Its rent and try time OP. This is a specialized kind of photography, and if you do need the very fastest apertures you need to understand what that does to IQ before you buy to avoid disappointment. Fast wide lenses are a category on their own, very hard to make, and requiring understanding when evaluating their results. Its dead easy to make a sharp fast 85mm lens, its horrifically hard designing a fast wide, with inevitable compromises in design and performance.
Rent and test.