Makes perfect sense to feature Superman in this movie.
Shazam! drew in the younger crowd, was light hearted, had family vibes... superman has the potential to further that audience for his own, I mean, what kid doesn't know superman, what parent doesn't associate superman with that cheesy super hero feel.
Tweak what needed it from Shazam! for the sequel - reintroduce superman - show us superman being lighter, approachable.. mentoring Billy. Almost as if we have Billy be the Jimmy Olsen style character, spiritually. We see Clark interact and show him his DP persona, how he does it and why.. we see superman through billy's eyes as he meets him.
Then, this movie sets up superman for a new set of movies and we have seen the evolution of sorts from snyderverse to more mainstream appeal.
The last thing they would, altho it is WB, is to have superman yet again be the opponent!