The appeal for me was the "it just works" factor. It's a compact unit and setup was easy. Every self-built solution would either be rather large (factor for me) and more difficult to set up. And I think, that's what has kept Synology alive for so long. It allows entry level users to get into the selfhosting game with the bare minimum you need, especially if transcoding (Plex/Jellyfin) is mentioned.

As an anecdote, I've had exactly this problem when buying my last NAS some time ago. It was DS920+, DS923+ vs. QNAP TS-464. The arguments for QNAP were exactly what you write. Newer chip, 2.5G NICs, PCIe Slot, no NVMe vendor lock-in. So I bought the QNAP unit. And returned it 5 days later, because the UI was that much hot garbage and I did not want to continue using it.

Lately, the UGreen NAS series looks very promising. I'm hearing only good things about their own system AND (except for the smallest 2-bay solution) you can install TrueNAS. It mostly sounds too good to be true. Compact, (rather) powerful and flexible with support for the own OS.

As the next player, with mixed feelings about support, the Minisforum N5 Units also look promising / near perfect. 3x M.2 for Boot+OS, 5 HDD slots and a PCIe low-profile expansion slot.