Someone told me to use proton calendar, and I have been. It doesn’t work offline. Like when I’m offline, I can’t add new events. I would like it if I could add new events, and then when it got connection, it would sync.
I think I tried tutanota a few months ago, can’t remember what happened but it didn’t have what I was looking for.
Look into Nextcloud calendar, you can use the default calendar application on your phone / desktop while offline and it will automatically sync with your Nextcloud when connected to the internet. Hetzner Storage Share is an inexpensive Nextcloud host with calendar enabled by default.