Nextcloud is snappy. CPU utilization is minimal. RAM is basically empty. I am happy!
You still need a case and a picoPSU, but those are cheap and allow you to properly fit all drives.
I boot off a small 120 GB SSD (slow SATA3), my media lives on a large HDD (slow SATA3) and all docker services, user data and databases are stored on a 2x 1TB SSD RAID (fast SATA).
There’s a free PCIe 2.0 slot on the mainboard which would allow me to add 1 fast SATA3 or 2 more slow ones. The slow ones are ideal for spinning drives.
Both devices you listed are 6000 chips and should be even faster and more efficient. Do they have PCIe 3.0? That would make them twice as fast I/O-wise than my setup!
BTW, stay away from USB HDDs and stick to directly attached SATA or NVMe. Much less finicky, reliably show up after reboot (!) and have SMART statistics! Also, external enclosure are made to look nice but heat up instantly. They degrade performance and life span.