If the others suggested aren’t quite right for you, you might try looking for an interval timer app. These are generally used for fitness, but it seems to me that type of setup might do exactly what you want if you just set up a “workout” that has a single 30 min interval and repeats.
Thanks. I just went and disabled it. I also found that they had “products and services notifications” turned on. I know I attempted to disable all advertising and monitoring stuff shortly after I signed up, but I can’t say for sure whether I had missed this section at that time or if they kindly turned it on for me between then and now.