Oh, isn't that interesting. I already run a Pi-Hole but I had never thought of doing that with a custom entry. I suppose that would work, wouldn't it?
I really dislike that everything in this thread is pushing towards Nextcloud after all. It seems like it should be easier to get email reminders. It should really be somewhere in-between "use Google" and "run Nextcloud and buy a domain name and use a custom piece of hardware to run your own DNS server to which you have to add even more customizations to be able to run and sync the Nextcloud suite, most of which you'll never use."
Unless I'm missing something, there doesn't seem to be any functionality for email reminders either on the server side or client side for them. I tried Etesync once, although it has been a bit.