Superman (2025) started filming in February 2024 and ended in July 2024. And then came out a year later.
Anyway, Gunn said that he's finished with the "treatment" (outline of the script) for the sequel. The main actors are also already cast, so there's even less work to do. He just has to write the script and then start filming.
The physical stuff, sure.
But they approached him in 2018 or some shyt, then he had ideas but didn't feel like he could do it until late 2022. That's like 4 years of mulling it over before he actually felt confident enough to build on it. That's where ideas get scrapped and others gestate properly..
With this timeline they'll pretty much be forced to make whatever treatment he just now came up with, with minimal time to course correct.
Not saying it's not possible, he could've spent the 5 years already thinking about sequels as well. I think the track record for Hollywood, and especially Warner Brothers, fast tracking movies isn't great though.
Again with minor Jor-El quibble aside I am all in for this Superman flick, so I'll be there day one for the next one.