same thing happened to me. talked about very specific health-related topic, didn’t googled it or anything, the next day instagram came up with ads for the very same specific health subject.
for a start, you can check instagram’s permissions (and facebook of you use it, it’s meta too) and disable access to microphone.
however, that leaves whatsapp which is also meta and mic is more of a neccesity than insta/fb, but in my case, haven’t seen weird ads for subjects i didn’t typed into any devices since i disabled insta mic access.
yes. use wifi instead of sim-card data and you’re all set. i doubt google play services care which way of data transfer you’re using to access your account on play store.