Preface: I don’t care if the data resides on googles servers. The goal is to be able to see/edit/add in nextcloud with the new google assistant able to make changes. I’m still looking into this as I just started to use google assistant.
In particular contacts & calendar and possibly notes/reminders.
Email is hosted separately from google, so doubt it will be able to work on that aspect.
Nextcloud has been designed to act as a safe home of your data, independently of Google and Co. Therefore I couldn’t imagine that someone is actively working on such an integration.
It’s calender/contacts and notes.
And I do understand you but it prevents Google, Alexa and Siri from working and I’d really like to use their capabilities. Last I checked there was no nextcloud assistant.
I’ll look into it and see if there are ways around the limitations.
I have no info on this.
However I have looked into this as well a while back. The problem is google does now work outside there own calander you can not have google assist alter a icloud contact or calander nor any other.
I personally disabled google contacts and calander sync on each device . And use open sync and open task apps for contacts and calender with NC.
As I mentioned no help. Keep us posted may you venture in a solution.
The Thought Police doesn’t like when you hide information from the Telescreen 
Not for Nextcloud (nothing was announced) but I think there are projects out there. Even one of the big players wanted to push more of the processing onto the device itself so that the data is not sent to the servers for more privacy.
On Apple you can manage your contact and calendars directly with Nextcloud, so Siri would probably handle these as well. Perhaps you should ask for native caldav and carddav support on your device.
You could subscribe to your Nextcloud calendar in Google Calendar. It’s slow to update and you might need to make the calendar public for it to work (forget if Google Calendar supports credentials for calendar subscriptions), but it works.