How much storage do you need?
Do you care about latency?
Do you care about throughput?
Do you have a total budget, or a total and per terabyte budget?
What’s your use case and which OS?
e g. There are also external USB 3.2 10Gbps enclosures that’ll fit several HDD … and there’s also raid cards and backplanes you could use to stuff your am5 system with dozens of 870 qvo sata drives.
Linux. Going to run 7950X pass through for Windows usages.
Latency not a huge issue, but it seems like orders of magnitude slower to drop to SATA?
Probably only need 8T storage. Buying an HDD will not fit into an mITX case. But if I am spending $800 two 4T NVME drives, and alternatively I can buy a 16TB HDD for $200 seems like I might as well build a reasonably fast external enclosure or a NAS with the difference. NVME prices seem unlikely to enable even $400 8T sticks in the next 12 months.
Interested to learn about all possible solutions. Pointers to guides extremely welcome.
Build your own NAS and stop being a pussy. You can get an old PC, a tower of some kind. Chuck a couple of hard drives in it. Get one of those three packs of USB3 drives. Make one the TrueNAS core installer, use the other two as mirrored OS drives, TrueNAS Installer offers you this. Once installed set your couple of hard drives or more as the ZFS storage and away you go.
Having an actual file server separate from your computer allows you to abuse your computer without risk of losing your data. You never touch your file server, leave it alone, it does not need to be interfered with. It will run for months without a reboot and years without a problem.
There are some great deals on ebay for old SAS dives which you will need a card for or keep things simple and just buy two huge SATA drives.