I am using Nextcloud Calender + DavX5 (android) + etar (android) for my colaborative and synchronized calenders. Working good since 1 year.

TL;DR: an app won't solve your problem, but learning to organize yourself will

At the risk of overstepping, I'll say this:

If you have a problem with organization (or lack thereof), a calendar app will only help you so far. You can still miss an appointment by swiping away a notification or neglecting to jot down the appointment in the first place. Software are just tools: they won't magically transform you and your habits.

(Disclaimer: I'm not a productivity expert or anything; I'm just applying what I've learned so far in life.)

First, think about your current internal system of keeping track of things. How is information getting lost? Why is it getting lost. From what I've read, disorganization is just a symptom of a deeper, underlying issue. In my case, I just didn't like thinking about things I didn't want to do or didn't consider important, so I didn't bother noting them down. So how did I remedy that? I have a notetaking app (Simple Notes or Markor) that's empty by default and placed in my phone favorites bar/dock. As soon as I need to jot down something, it goes there. Once I can take my time, I review what I've jotted down and decide my next steps. When the tasks are done, the app gets cleared.

Second, think about what has worked for you in the past. Are there things that you managed to remember, and what helped you remember them back then? Personally, I remember things when I see them often, so writing down tasks in my notetaking app already works. However, for things that have set or faraway schedules, they get moved to apps with persistent notifications (Tasks.org and Simple Calendar, synced with EteSync).

Third... rinse and repeat. You may find that your new system still has gaps. That's just part of learning how you best organize. Don't be afraid to experiment, but don't forget, and I cannot stress this enough, that SOFTWARE ARE JUST TOOLS. If an app doesn't work for you, then it just doesn't fit you, period. It's not a failing of the developer or you. Once you find an organization system works for you, you'll find that it's so natural that it's just second-nature. What apps to use will come after.

I'm sorry for the long comment and that this wasn't what was asked, but I felt that this needed to be said. I do hope it helps you in some way.

If I was trying to save a relationship, "FOSS and encrypted" would not be my priorities.

Just use whatever whatever is easiest for her.

It's likely you need much more than a calendar, though. Here are some more suggestions: https://www.chorebuster.net/blog/2023/06/free-cheap-apps-for-managing-adhd-around-the-home/

If self hosted why does It need to be encrypted? It only protects from physical attacks

To respond to your direct question, Tutanota has you covered.

To respond to your broader organization predicament, I think you need an organization system. One of the most tried and tested systems is Getting Things Done. Check out the workbook. I'm sure it'll change your life for the better. https://www.youtube.com/watch?v=4aD8WG49PY4

I could send you some GTD resources https://matrix.to/#/@blipiblupi:matrix.org

Of course, you can get them online as well, given how popular GTD is 🤷

Proton Calendar works well for this, only thing is you'll need their unlimited 2 year package for ~$200. Comes with 10 VPN connections, upgraded simplelogin account, shared calendar, 500GB of enceyed cloud storage and many email perks.

Maybe: https://www.etesync.com/

But e2ee and sharing with other users is always complicated.

Tell her 걱정 마 다 괜찮아

Proton Calendar, you can add "participants" to an event and it's E2EE and free (with premium paid plans, but I dont think that you are going to need it). Worth a shot.