You’re really best off not trying to lump together your Plex server and your NAS on the same box. You can use an odroid H4 or any cheapo Alder Lake-N laptop for your Plex server, and any NAS solution to store the media.
Tbh another archaic idea is that you need to transcode.
All you need is one of those sticks that plug into the hdmi of your tv and can run vlc. I use a chromecast. Vlc can play mkvs directly off samba shares. Done.
TBH the only reason anyone talks about transcoding is Synology's absolute dog-shit hardware options make it a problem. Most Intel CPUs and most AMD CPUs and many ARM CPUs and all the discrete GPUs are absolutely fine with munging video and audio codecs. They're just not well-represented in Synology's lineup.
Fundamentally I guess it feels like they have ground to a halt or even quietly retired the DSM line.
Let's look at your NAS for an example (it's actually worse for mine cause I always loved the Slims lmao).
DS1821 has no GPU transcoding because it uses the AMD Ryzen 1500B, what you've observed is this processor is just powerful enough to brute force some transcoding but it has no hardware transcoding:
DS1821 successor was recently revealed to be the DS1825. So if your NAS died in a few months, that would be your obvious choice for a replacement. They decided to continue using the AMD Ryzen 1500B, which is now seven years old. TBD if they bother releasing it this year. In a few months they could call it DS1826 instead.
Meanwhile DSM still lacks support for NVMe volumes, in fact there are still no NVMe-only models at all, you can't even install DSM to NVMe, they cut support for USB drives, they cut support for HEVC, and they almost never update docker and some of the other tools.
The hardware's going nowhere and the software's going backwards.
I would like to use ssds for running containers, this is a good point. The elderly cpu is more than adequate for my needs but as you say, an upgrade, ever, would be nice.
I guess I’ve avoided these issues by accepting that solid state storage in a NAS is still too expensive.
My ultimate would be the Synology with an Apple silicon heart. One can dream.
That said, the person you're replying to is right. Synology has mostly stopped supporting their apps, they've removed features in cost-cutting features (media codecs), hardware is now hopelessly outdated and both kernel and docker are completely out of date.
It feels like any technical leadership completely disappeared and now only bean counters who don't understand the product or their target market are making decisions.
You got different replies from others, but my 2¢ since you were replying me - if you're not doing transcoding, then I agree, Plex on a NAS is fine - it's basically just a wrapper around some shared files in that case. (The post I was replying to did specify that they were doing transcoding.)
Even if you do transcoding, it's still fine so long as it has supported hardware for HW-accelerated encoding. Which doesn't require all that much - even fairly basic Synology models can do H.265.
Some do, but the models with HW-accelerated transcoding have been shrinking - it's only the "prosumer" models that have Intel Quick Sync (with no guarantee that refreshed products will maintain it) - the lower end models have ARM processors without HW-accelerated transcoding and higher end models have Ryzen processors without GPUs.
About a year ago ago I bit the bullet and frankestiend together a TrueNAS scale box. I hated the idea of the DIY route and stuck with frustrating SOHO nas devices for way too long. An all in one NAS product that has an app store or supports containers is nice in theory but in my experience always ended up with a NUC running proxmox sitting next to the NAS.
Managing it is fine. It expects you to understand ZFS more than a turnkey RAID 5 + btrfs job, but has an OK UI and seems born out of the fact ZFS people want that customization, not forcing you fend for yourself. I read a 15 minute explainer, built a pool, and didn't have to think about it at all other than replacing a failed drive last month. And all that took was a quick google to make sure I was doing it right, which is exactly how I replaced drives in a standalone nas.
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.
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.)
I saw on level1techs, a 4 Bay chassis with an AMD APU plenty capable of transcoding multiple streams. Can't remember who made it, and you'd have to DIY the OS install, but that's pretty ideal to me.
What i do, and will continue to do, is use a USB-C disk box (Level 1 also recently reviewed some they quite liked, despite the usual fears around USB) and whatever PC i have laying around. 5 years strong running ZFS over USB 3 with a 4770K, regularly serving 4 Plex streams at once without complaints and no failures (i mean, usual disks wearing out, but nothing caused by USB).
So if a 4770 can transcode 2x1080 and direct play 2x more, any old anything with hardware transcoding these days should be just fine.
Assuming I want 4 drives and something that can transcode multiple files in real time.