I think the lens(Sony 16-35mm GM) may have taken a hit somewhere, it does some funky things sometimes, a few photos had some weird focus issues. I shoot maunal for for 99% of landscape shots and use the focus magnifier to get critical focus. It's out of warranty and I don't know how much Sony would charge to service it. Would love to get the new 14mm GM and 24mm GM primes to replace it but that's a lot on money.
I would suspect too long of an exposure, it would explain the focus because the water would be moving and the rest of the scene is static. But that is just a guess.
You can always rent a lens to try it out.I shoot Canon and I really want a RF 24-70 F2.8, but holy $$$. I take photos for me, so I have no revenue stream from it... Makes it much harder to justify.
I have the Sigma EF 100-400. It's a great lens, but I'd like to get native and they (Tamron or Sigma) don't make RF glass yet. The Tamron 24-70 is 1/2 the price though... The Canon EF to RF adapter has been sold out almost a year though and my off brand adapter isn't great.You can always rent a lens to try it out.
Canon makes great glass, maybe check out Tamron and Sigma, they’re improving their products all the time.