Nextcloud is my best pick. Of course, it does much more than calendars, so if you’re looking for a calendar service that does only that it may be a bit overblown.
Big pros:
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Native WebDAV integration out-of-the-box - which means that you can easily export your calendars through WebDAV URLs in any supported app
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Support for importing external calendars
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If you also use other Nextcloud products (like mail, contacts or office) the integration betweren them is quite well designed
On Android then I use DAVx5 to synchronize calendars and contacts from Nextcloud with my phone, and Etar as a multi-provider calendar app.
KOReader