Self hosting a calendar isn’t too difficult. I use NextCloud personally, but as the saying goes “it’s overkill for your specific requirements”. Nothing wrong with overkill, though.

Another option might be LubeLogger. It’s designed to track car maintenance, but you can set up time based reminders. For example you can create a “vehicle” called Dishwasher and set a once a month reminder. The problem is notifications… LubeLogger only does email if you set it up. I hacked together Ntfy notifications but that was one of the more difficult things I’ve ever figured out. It’s possible, though.