This seems like an odd question, since the alternative is “no backups at all”, isn’t it?
Realistically, I think for a handful of iDevices 2 TB is going to be plenty. Some app data isn’t backed up. For example, I have Logos Bible Software, and I have probably 50 gigabytes of data downloaded. That’s not in the backup. I have probably 20 GB of data in Overcast. That’s not backed up. When you add up all that stuff, it effectively means my backups - even on a 256 GB iPhone - are relatively small.
So I wouldn’t be worried about backup space in the short term. I would potentially be worried about a family iPhone taking an unplanned bath and all of their saved media vanishing.
I reduced my online storage needs by removing music and movies I had ripped from physical media.
I still have a local copy and the original media. Most of my movies are available to stream, usually for free. And the music I upload to YouTube doesn’t count against my Google Drive storage.
Does iCloud save device backups to the same cloud where general iCloud data is stored? I’m thinking of the obvious point of having your data and backup in the same place. If something broke with iCloud, would that compromise not just icloud data but all the device backups also?