I have three different calendars syncing using caldav, one on fastmail and two on icloud. When I open the calendar view it's often the case that one or more of these timeout (all of them are afflicted by this), so it seems that these calendars are not actually stored on the server but polled everytime I want to view them.

Are there any alternative integrations that will periodically sync the calendars and keep them on the server? Or can I self-host an app that does this and will never time out because it's on my local network?