I haven’t done a lot of Astro, mainly because there isn’t much opportunity where I live, but it is an interest of mine. That said, I can tell there are no worries using an adapter and an EF lens.
Adapter is essentially an extension tube with straight through contacts, no real electronics to speak of.
As far as lenses, it seems you have ruled out some of the long recognized Astro champs. Personally I think the variability concern on the Roki and Samyang lenses is overblown. While valid a few years ago my experience is the problem has been resolved. I’ve had Roki 14mm manual and 85mm manual lenses in EF, 12mm manual in both EF-S and MFT, and now have Samyang 85mm AF in RF. Exactly none of them have had any issues.
I don’t think I’ve ever seen a review that didn’t mention “some” coma in corners and it seems no lens made is coma free. Now some certainly are much worse than others, but in this case I don’t think perfection exists.
the RF 15-35mm is getting good reviews for Astro if you really want to avoid the lenses you mentioned. I have the RF 15-35mm and it is an amazing lens for landscape but I have not done any Astro with it. When I have a chance, it is what I will use.
if I were in the market to buy an Astro lens, I would pick the Samyang 14mm f/2.8 AF in RF mount. They changed the optical design when they started producing AF lenses, but they still get stellar reviews
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if you are a total peeper and concerned about coma, a 14mm lens on the R5 in crop mode is 22mm equivalent, still pretty wide and wil give you a 17+mp image. That should remove the corner coma concern (as would cropping in post).
Always run the risk of a bad copy, but buying from Amazon makes it near hassle feel to return/exchange for another.
