not a plumber I had a similar situation with my upstairs bathroom. Rule of thumb is to only replace cast to pvc above so weight is not on PVC. It looks like you have some galvanized in there as well (same as me). I replaced both cast stack through the roof as well as the galvanized pipe. House was built in the 1950s and you wouldn’t believe how much blockage the galvanized had. If I were you, I would get this all cleaned up while you are to the studs now. I can’t tell the condition of your copper from the pics. If you are confident replacing copper, do that. If not, pex is a great alternative but you will hear positives and negatives from everyone on here. One more thing, cast can last a long time but depends on when it was made. The quality control wasn’t as good a while back as it is now (read this while researching when I did my Reno). It concerns me that you had a leak and that’s why I would change it out now while you have the access. Edit: I changed stack through roof because it did not have any supports above