Radical on the server, Davx5 on the phone to sync, Thunderbird on your desktop. The combination has worked for me over many years.

Proton has a good calendar service but if you want to change color labels you need to be on the more expensive tier - and they don't really tell you that ahead of time. Which p'ed me off so much I moved my calendar to a Nextcloud server instead, which works great. It also got me off Keep, Drive, and I'm working on Gmail.

Nextcloud, self hosted. But it is maybe overkill, unless you need it otherwise, for family photo album, docs or such. Davx5 also needed on android. Thunderbird for desktop calendar client. Can recommend etar for android calendar app.

I self-host baikal and use davX5 on android with whatever calendar app i want, and on desktop i use betterbird but you can use any calendar app you want.

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:

• Native WebDAV integration out-of-the-box - which means that you can easily export your calendars through WebDAV URLs in any supported app

• Support for importing external calendars

• 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.

As others have said Nextcloud has a calendar, viewable via the webam or thubderbird (can also use the tasks) and any floss calendar app really on android or even the one installed by default probably.

There are a few orgs that permit creating an account so you can avoid selfhosting. Some like from Framasoft are donation funded.

What I use covers not only calender functionality but also address books:

Server: I recommend Baïkal. It's based on the same calendar and contacts library that comes with Nextcloud. Reliable and powerful. This of course means you have to host this PHP app somewhere. If you prefer to have it as a service, a small rented Nextcloud instance may be easier to get because of it's popularity.

Android: As already mentioned DAVx5 covers the sync function very well. As for the Calender app I like aCalendar+ but many others (maybe even stock calendar, but I've never tried it) should work. For contacts the stock app works here, as do others. One thing to look out for: You now have multiple calendars/address books. Make sure you store your data in the DAVx5 based ones. Check if your apps allow you to actually select them, or even better to make them the default. Otherwise it may look like sync is broken.

Linux: I use Thunderbird which supports CalDAV and CardDAV natively.

I use these for years without problems.

Tutanota calender app. Works offline. Encrypted. Sync with tutanota login so very easy to setup.

Use tutanota webb to view calender on pc. Might work as well with their desktop app(not tried)