A few things.
First, if you're not viewing at full size or anything close, problems with coma, chromatic aberration, and astigmatism are less important. Resized to average web viewing can cover a lot of sins.
Second, as Nubu said, you can use tracking mounts with reduced speed if you're shooting foreground/horizon too. I've never used the 1/2x speed on my iOptron SkyTracker, but others have with good results (within reason of course, especially at reduced size).
Third, the Samyang/Rokinon f/2.8 is a wonderful lens for daytime and night ultra wide angle work. You can't get front-mounted filters for it, but I was wondering what kind you meant? If astro filters, the slot-in type (for crop sensor) are good (this is why I use it on the 50D). Other wide angle lenses may take rear-mounted gel filters.
One thing not widely discussed is ability to focus. I would use the 24-105 f/4 for astro work, but it is too dark and the magnification too low (even at Live View x10) to get accurate focus on stars at 24mm, so has never worked for me. You can try different manual focus points through trial and error - I found the Samyang's less loose focus ring easier for this. But with a wider aperture lens, the brighter image can help with focus.
Stopping down isn't always worth the effort. I have experimented with the 85 f/1.2 II at various apertures, and I'm not sure the benefits of less vignetting and distortion at narrower apertures exceeds the loss of light. Do look into flat frames, which are master images taken that can be used to cancel out most of the colour and vignetting problems (but not coma etc; NB it can be hard getting flat frames at the shortest focal lengths).
One last thing. It may be cheaper and easier, depending on where you live, and if you drive, to go to darker locations. Most of what I do in astro work is fighting light pollution. If you can go somewhere dark, any camera will take good wide angle astro images. Good luck in any case!
Below: Milky Way from suburbia, Samyang 14mm (70mins stacked + tracked, no darks/flats).