That sounds great! I plan to build such a reminder service for myself with additional infos like the subscription costs.
How did you managed to show different categories/calendars and how did you do the repeating scheduling of events?
I just create a single calendar specifically for my app, then add things to it directly. Calendar events already support all of the repeated scheduling I needed. I didn't setup anything for categorization directly, but could be pretty easily done by prepending an emoji to the event name.