They say time is is the most valuable resources. Right now, time feels quicker for me these days and I often lose track of it.
Because of that the app should have the same purposes as an old clock, it plays a little “ding” or a notification every 15 minutes, 30 minutes, or 1 hour, or as long as I like.
Preferably installable with f-droid, can I have an app recommendation.
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.