I have a Hikvision NVR and 4k camera setup. I keep them on a separate subnet and have blocked all access to other subnets and the internet via a Mikrotik router.
In the year or so I have had this setup there hasn't been a single packet from the NVR or cameras in an attempt to access the internet.
The way I do it is: When I leave my home my home assistant will enable the FTP upload on motion detection feature of the camera. If there's motion then the camera will upload the recording to a ftp server running locally (synology), this then gets synced to google drive.
Home assistant can also do push notifications if there's movement detected.
for example: if you want the NVR to upload to Dropbox, you would whitelist the list of domains that are needed for Dropbox to work. if the NVR tries to connect to 'heartbeat.hikvsn.cn' it would fail.