How about a shared calendar you both subscribe to?

I'm not sure self-hosted is what you need, perhaps just a FOSS app for your two phones?

Alternatively Home Assistant should be able to push alarms to your phones.

I too would like to know. I use the app Due on iOS. And it is nice and not a sub (though after 1 year you technically need to buy it again IF you want features that released into the app after 1 year of your purchase. It’s also iOS only and doesn’t let others force a notification on you. But if they have the app they can create a notification and text you a custom link to auto add that notification to your app. For anyone reading who is on iOS and wants something like this. due is really cool. Notifications but with a lot of customization. Most important feature is that you can use critical alerts. So like an alarm it can override silence mode or focuses to notify you every X minute until you snooze or dismiss. Can also disable snooze but then I find my self dismissing without actually doing it.

Either way a platform agnostic proper notification system sounds right up my alley. Just googling around I found Apprise and Mind might look into that later (don’t know nearly enough to say if it’s a viable alternative to Galarm or Due but seems extensive and lightweight.