2 HDD Storage Strategy on remote install

Hello everyone,

I'm remotely helping someone (let's call her Amelie) set up a PC as a NAS using OMV.
She already managed to install OMV on a separate SSD.

For storage, she got two 4TB drives. She says that she won't need more than 4TB, but we all know how it works. What would be the best option to set the two HDDs up? What I came up with:

a. a simple BTRFS formatted drive for storage, use SnapRAID to ensure data integrity in case of a drive failure. This solution allows potential drive expansion, but needs to be run manually and I expect her to forget to do it. A cronjob might be an option, but someone needs to run and check for

 snapraid status

from time to time, which again, it is very possible she'll forget after a couple of times

b. a BTRFS mirror raid. Data parity is automatic and it would be easy to recover from a drive failure, but it would be more difficult to add another drive in the future.

I read that BTRFS arrays can be expanded, but since it's a remote system, and I have zero experience growing BTRFS pools, I wouldn't go that route.

How would you tackle this situation?

I know that RAID is not a backup, and she already has all the important data on a separate USB drive. I was looking for the best way make use the two drives without compromising any possible future upgrades.

The mergerfs + USB backup plugin sounds like a great idea, we'll try that.