I recently went through the process of separating from Google as much as possible here.

As others have said, Nextcloud or Radical or Baikal are all good calendar server options to self-host

On your Android phone, DAVx5 for syncing CalDAV and CardDAV (which the servers listed above use), ICSx5 for any public Google calendars you want to subscribe to (you can almost always get an ICS calendar file link for those), and Etar to interact with said calendars on your phone.

On your computer, Thunderbird is the easiest way to go. There is also the web interface for whatever server you decide to host. There are other options, too. On Linux, I use pimsync + khal/khard.

Caveats:

• In Etar, khal, AND the Nextcloud web UI, I have had lots of trouble with being able to apply updates to calendar events, like a new ICS file containing an updated time or place. The only calendar app I've found that handles this correctly is Thunderbird on the desktop.
• Doing things this way separates your email account from your calendar account, which can create some annoyances. Every mainstream mail service these days tightly couples itself with a calendar. For instance, to send invitations for a calendar event that I create on Nextcloud, I also have my email account linked to Nextcloud. You can't do this if you have Proton or Tuta because of their encryption. When I had Proton, I used Postmark to have a send-only email account from Nextcloud to send out invitations.
• If you want to subscribe to PRIVATE Google calendars (my partner still has Google, so I need to do this), you need to sync a Google account with each device you want to subscribe to that calendar on. There is no way to add it to one of these self-hosted servers. The way that I handle this is by making a throwaway Google account that is only for subscribing to calendars, syncing that to my phone with DAVx5, and while I could sync to Thunderbird on my computer, I pretty much only use Thunderbird when I need to update a calendar event. So, in my case, I use pimsync. To sync a Google account to pimsync, you need to create a fake "app" using a Google account on the Google Cloud Platform, add the CalDAV API and generate credentials, add your calendar sync account to the allowed testing users, and then add the generated credentials to pimsync. It sucks.
• As usual, you can't sync any Microsoft calendars with anything other than Outlook.