I do think the Flash movie will be used as a way to smooth out some edges in regards to canon but not with Superman. Henry Cavill is still contracted for one more appearance and the Rock seems very interested in having him in a Black Adam movie.I have this weird feeling the Flash movie is gonna start with Synderverse Flash but at the end he's in the canon universe where Justice League happened as in the Whedon movie with one huge exception...
"Who's Superman?"
Setting up the JJ produced movie.