> I'm even getting my cooking gas as 'prepaid' (i.e. I buy bottles) and I wish I could do the same with electricity and so on.

Gas and electricity do have small fixed components (“hookup fees”) but for the most part these aren’t strictly subscriptions since you pay for what you use.

> So I'm totally fine with getting a gift card or whatever you call those thingies in the supermarket where you can throw a lump (20€, 50€) at some provider (Google Play, whatever) and then get to eat up that amount (over the course of at least one year, hopefully).

So you are okay with subscriptions as long as they won’t charge you more than X? You might like privacy.com. They let you create credit cards with unique codes and defined spending limits. You don’t need to worry about cancelling anything since your billing just won’t go through after a certain amount is spent.