You can configure “Thunderbird” or “Evolution Mail” for a deeper integration to the Gnome desktop environment. Contacts Calendar
The nextcloud-desktop app works fine. I’m using Debian which has a working “nextcloud-desktop” package in the regular repository, so i guess Ubuntu has it also without the need to use a snap
Ubuntu/Gnome has a native integration for Contacts+Calendar, just go to Settings => Online Accounts => Nextcloud and enter either e.email credentials, use another hosted Nextcloud or your own. Not sure if gnome-contacts and gnome-calendar is part of Ubuntu, but I think so. Evolution integration is seamless and won’t need additional steps if you receive .ical Mails.
For file sync it’s the the official Nextcloud App. If you’re not yet familiar with Syncthing, give it a try, it’s p2p within your own network, shuffling gigabytes around. For anything you’re sharing with yourself and not potentially friends.