Even though you said “isn’t Nextcloud”, I’d still say it’s perhaps the simplest solution.

You can disable most the other apps and set calendar as the landing page. If you don’t use the other features, the resource usage is very low, just a cron job that does basically nothing. I don’t think disabling the default apps has much effect on the footprint, by the way.

Calendar, contacts and notes are why I still self host nextcloud. Just remember to pay/donate to Davx5, they’re one of the projects that need to keep running!