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As long as you don’t know how fast your storage requirements will grow, you can’t make a rational decision.
So do something temporary, until you do know.
Get one 16TB drive for your bulk storage. Use your other HDDs for backup. When the backup drives are full, get another 16TB HDD. Make a new decision later.
Nope. I just didn’t mention it. Before I got double SSDs and double DAS, I already had a remote NAS, cloud, a laptop, various external drives and storage on my phone and tablet. It started with the laptop, when it was new. I installed a NVMe SSD and a SATA SSD in the laptop and setup automatic backup of the NVMe SSD to the SATA SSD. And that worked so well that I then I did the same with my PC. At one point I had two NAS. One Synology and one DIY RPi4 based with a RAID enclosure. I reused the drives in the 4 bay RPi4 based NAS for a DAS. Worked so well that I got a second DAS. Still have the Synology NAS, but at a remote location.
You already have backup software installed. Just use it.
Possibly with an external drive. An external multibay DAS will give room for growth and reduce cable clutter.
What I did was to install two SSDs in my PC. One is only used for backups of the other, every boot. (Hardlinked rsync snapshots.)
In addition I got two multibay usb DAS. I use one DAS for media storage and backups of the PC, and one DAS for backups of the other.
Seeding will wear out any HDD quickly. Use a SSD instead.
Sequential access is where HDDs shine.
Random access is where SSDs shine.
I am very pleased with my IB-3805-C31. A 5 bay USB (10Gbps) JBOD enclosure.
You access the drives as separate drives and can turn them on/off individually.
I use it with Linux and mergerfs and snapraid. Works very well. Silent and robust. Sabrent has an identical(?) enclosure.
It is likely that the files will be fine for years. But they may become corrupt tomorrow.
You need multiple copies, on different types of media. And check/migrate regularly.