You are way overestimating how much time it will take.
I want it because I think it is a cool moment that does nothing to take away anything from Shazam but builds up the universe more. Do you not want it because it is Superman?
The Wizard spends all his time in the Rock of Eternity. He doesnt do much training unless Billy visits him. I highly doubt that will happen much.
I don't think so. David Sandberg made an instagram post about how movie making takes way more time and effort than people realize. If they use Superman, they have to consider a lot more like reshoots and things like CGI mustache after that happened. We shouldn't assume to know how much it cost or how much time it will take because that's something they will have to figure out. It's not that simple.
The thing is the movie doesn't HAVE to have Superman. That's just something you want. You might as well include any other character. There's no reason to put in Superman because he's not important to Shazam's origin. It would be something extra that they would have to insert and make work. To be honest if they were going to do a cameo, I rather they put in Cyborg because they're buddies and he would fit the youthful vibe better, but I rather have no big cameo scenes.
The Wizard usually speaks to Billy and gives him guidance. Between the Wizard and Freddy, Billy has all the guidance he needs to start off.

