I switched to Synology about six years ago (918+). The box is small, quiet, and easy to put in the rack together with the network gear. I started with 4TB drives, gradually switched to 8TB over time (drive by drive). I don't use much of their apps (mostly download station, backup, and their version of Docker to run Syncthing, plus Tailscale). But the box acts like an appliance - I basically don't need to maintain it at all; it just works.
I don't like all this stuff with vendor lock-in, so when the time comes for replacing the box, what are alternatives on par with the experience and quality I currently have with Synology?