Going off on a slight tangent... bit first.
I bought a Laowa 12mm F2.8 and absolutely love it. I've really been concidering buying their 15mm F2 (although that is around £700) Could that be a half way house between your 16mm and the RF L series zooms? Not sure how the picture quality compares to the 16mm RF but I'm more than happy with the 12mm produces some fantastic perspectives and good colours. I've used it for Astro too and had few problems with the corners. It is a manual lens though, and at 12mm I just set it to infinity if I'm out in field. Not sure this would work quite as well @ 15mm but could use focus peaking etc
On that subject my 16-35 2.8 is the Mk2 version and the corner sharpness for astro is horrendus. Has anyone used the later mk3 for astro and what are the corners like? Might see if I can trade and find a decent price mk3 if better, as it would be nice to have a lens I can use for Astro which takes regular filters rather than square ones.
I think this is only photo of the stars I ever took with the 16-35. You can see the problems with the corners on the left, how the star shape changes as you move left to right towards the centre
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