Cloud volumes ONTAP actually means to run ONTAP in the cloud. that will cost you in storage space, compute and licence. that high use of compute - maybe more fit for DR, so it is available at anytime (with no need to restore).
NDAS should be available in a few weeks. according to the Demo - tha's seems like the right solution for backups in the cloud. and the only compute resource it seems to require is just to run the web interface and orchestrator.
There's an option for you to tier the current backup system to the cloud with fabricpool.you will still need your physical system to access the data (as the fabric pool files in the S3 will not be readable by any other system). if your snapvault system is still under support - that will likely be the most cost effective way to expand it if it reached the hardware limit. you can see this scenario on page 8 of https://www.netapp.com/us/media/tr-4598.pdf
i can suggest another option. and that's to find a 3rd party with NetApp that will do the snapvault for you. i know two vendors in the UK that doing it. but there's likely more.