Can’t answer question one as I don’t use Fairphone open.
So regarding question two:
Update of your rooted device through updater app is IMHO not possible.
One possibility is to manually install the current version (see e.g. https://support.fairphone.com/hc/en-us/articles/207914363-Manually-install-Fairphone-OS-for-the-Fairphone-2 ) and root your device again after the update.
You may plan to do future updates as described here:
[HOWTO] Root with Magisk - OTA updates, root cloaking and SafetyNet
EDIT: and I recommend to do a backup before the upgrade!
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