I run EXOS drives in the under-stair cupboard. They’re noisy but they’re not that bad.
There’s definitely a chance my knowledge is no longer current but I would 100% verify that for your operating system of choice (which I presume is Linux), your AMD CPU can deliver hardware transcoding under Plex. I’ve not heard of AMD CPUs handling this under Linux at least. Ready to be corrected.
You’re not using a CPU that most distributions support for hardware transcoding. You either need to use an Intel CPU with QuickSync or stick a discrete nvidia card in the box. The Intel route is often easiest here, and I say this as a die hard AMD fan.
Are you intent on building your own box?
I’m only asking because TerraMaster does the F6-424 (or F4-424) series which has 6 bays (or 4), a decent CPU (1235U) with hardware transcoding support, space for two 4x4 NVME m.2 SSDs, which runs silently and will just work as an appliance, even though it is a full PC. You can then install unraid or truenas on it, or heck bareback Linux and do it yourself. There are decent alternatives to putting something together yourself.
Regarding disks Seagate EXOS are often cheaper than IronWolfs and have higher MTBF than even the Pro. Don’t ask me why they’re lower cost, for more bang.
I’d just search for the specific CPU and “plex” and “transcoding” and see what comes up. I’ve not heard many success stories of AMD CPU transcoding, despite them having good hardware support. Stand ready to be corrected.