As for during the writer’s strike, I believe they could still film what was written. They just couldn’t write more or change the script. So I;d guess the idea might have been to film as much as they could, then do rewrites and reshoots after the strikes end.
Except they’re not? Filming was shut down as soon as the SAG-AFTRA strike started.
https://variety.com/2023/film/news/deadpool-3-stops-production-sag-strike-1235668880/
As for during the writer’s strike, I believe they could still film what was written. They just couldn’t write more or change the script. So I;d guess the idea might have been to film as much as they could, then do rewrites and reshoots after the strikes end.