What would an R5a really offer that the Ra does not for astronomy?
Eye AF is useless for astro work.
Faster frame rates are useless for astro work.
IBIS isn't useful if one is using a tracking mount.
8K video is useless for astro work.
Smaller photosites (i.e. higher resolution) aren't that much more useful for anything but planetary astro work.
I haven't used the Ra, so I'm not an expert on it's limitations. I would hope that it could take decent daylight photos after correcting the red channel for daylight use.
Regarding the points you made, I would agree in general, although IBIS might help for quicker lightweight tripod shots in windy conditions that are to be stacked later, or for handheld shots at twilight etc. And realistically, I could see them coming out with a R6a instead of an R5a reasoning that larger pixels are more appropriate for astro work and to get costs down so it sells enough to warrant production.
The one point that would be useful for me would be to have a 2nd body to pair with my existing R5. It would be able to take beautiful astro shots of the sky, as well as (hopefully) daylight photos once the red channel is properly adjusted for them. And almost all the controls, menus, and features would be the same as the R5. And for my travels when I take only one body, taking it would give me access to day and night shots. And it might have additional upgrades from the R5 so I might to use it as my 1st body.
For those that would be buying their first R body, or upgrading to a better body of R5 or better, they could get a great body and (hopefully) also one that would take great astro shots.
Now I do assume that an R5a probably won't be built, or else be built well after those like the R5s or R1 are built. I may decide to get the R5s as a 2nd body when it comes out - I'm undecided on that as 90MP isn't compelling to me as I don't really think the images will be much better than the R5 and I'd have undesirably bigger files to deal with - But it may have other appreciable upgrades over the R5, we'll see.