For in-app messages, shouldn't the content of the notifications be separate from the code itself? The code should be able to pull the latest notifications from some dynamic data store (be it a database, KV, redis, or whatever), just as a basic JSON or whatever. You shouldn't have to deploy any new code for that.
Apparently there's a SaaS called Courier.com that can do all this in one integrated system, but I've never used them.