Which shared calendar do you use?

I'm currently using a Nextcloud instance hosted by a volunteer run non profit. 12€ per account per year, I have one for me and my girlfriend.

However, calendar sharing is limited to people on the users on that Nextcloud instance, which is a bit limiting.

Do you have alternatives that allow federated calendar sharing?

https://calendar.online/ seems promising and is German, has anyone used them before?