there is still vali 3-2-1 approach
reduced up to your business.
Then if you will make first backup at primary site (different NAS) and second backup at second (different) site you need to take into consideration how to handle the backup scheme:
1. backup of data during their inactivity (traditionally during night time), if there is such kind of operation. then light two bay NAS with single ethernet is enough.
2. backup of data during their (data) activity, then you have count, that you will get two different backup (primary, secondary site) based on time between two of them performed. Then you need solution based on btrfs, that can help you minimize this risk. From two bay NAS it is only DS718.
Re: data grow plan. Not sure what kind of data you have in usage. I wrote here (in this forum)some helpful guide how to audit and calculate and plan future data space based on Win tool from Foldersizes.com, which is able make also automated snapshots during specific time to compare your expectations. But people are really surprised when they spend 1 hour with preparation of the calculation (over/under estimated). Definitely you can forget for wisdoms prediction (Gartner, ...), they change it every year without guarantees. Just your preparation can help you.
But if you need just max. 3TB for next 3years, it is good approach to purchase 4TB disk capacity (2x in RAID1), because in 2022 you will purchase 8TB for half of today’s price (maybe less). Then replace disks for new pair if needed.
Finally, any two bay NAS can help you. It is up to your backup scheme decision. Good luck.
PS: Seagate IronWolf is better than WD Red, but it is just my personal point of view.