For "just Plex" - more specifically anything except Synology's own suite of apps - any Mac, Windows or Linux computer or headless docker server will provide the same experience with a ton of better options, like installing to a NVMe without shenanigans for a start!

That all sounds like something that will require maintenance. Being able to slide a dead drive out, slide a new one in, and have any extra space be added automatically is fantastic. Not having to worry about updates, etc. These are things I'm willing to pay for.

If Synology is not interested in your use-case it doesn't really matter if you'd pay them to support it. But also they suck at most of what you're claiming anyway IMHO. Even their docker version is six years out of date.

How do you install an nvme in a Mac?

How do you expand the storage to 100+ TB?

The web gui is nice to use, what does Windows/Linux or the Mac offer? I’d prefer a Mac option, but I don’t believe there is a good one.

Upside of Mac is good Thunderbolt support. A ThunderBay 8 (or comparable) gives you 8 SATA bays. The baseline Mac Mini supports up to 18 such Thunderbolt bays via chained Thunderbolt. (Can also get nvme thunderbolt drives.)