What calendar app do you use and what address book?
Why I ask: Until now I still use an Apple iPhone and then use the browser-based icloud in ZorinOS to see my addresses and calendar entries.
Now I'm toying with the idea of switching to a phone with Android without google (e.g. LineageOS, /e/OS, CalyxOS or GrapheneOS.
And there of course I would like a solution how to get contacts and addresses synchronized on phone and laptop (ZorinOS) via some cloud solution.
I just use Proton Calendar now (Made by the same people who do Proton Mail).
Preinstalled Calendars and Contacts should be fine, on Calendars options menu you can use
Online Account… and probably Contacts does the same once started with its first configuration.
I use an /e/OS Teracube 2e Emerald de-Google'd phone, it's got the Murena email@example.com setup with calendar / email / contacts / tasks / reminders / cloud storage setup.
That's the web interface... there's also apps on the phone for all of that.
Here's how to configure a desktop app to connect to Murena's email servers:
I just discovered that my web host let me install nextcloud. In the nextcloud can be installed a calendar and an address book. I assume that there are applications under /e/OS that can access nextcloud and synchronize. Who knows?
I believe when you use the Murena setup I discussed prior, you are using NextCloud:
Scroll down about halfway to the title "Create Your Free Nextcloud Account with Murena".
Yes, Murena is nextcloud. I have just opened an account there. But I might not even need that if I have nextcloud with my hoster myself. At least I know him (a German, owner-managed company based in Germany).