Yes, this is what Plex does. You have a server in the basement, and, say, a Roku or a RaspBerry Pi or an nVidia Shield or an AppleTV hooked up to your receiver’s HDMI in.
It’s a bit the same philosophy as Roon: you have a big computer stashed away somewhere, and then you can spread small and cheap devices around to play things back (or superfast devices that don’t need to have big, noisy drives with lots of fans to cool everything down if you’re doing upscaling, but that’s a whole other can of worms).
The main difference between audio and video, as far as you’re concerned, is size: RedBook FLAC is, give or take, 300 or 400 megabytes an hour. 1080p BluRay is around 30 gigabytes, so, give or take, 100 times more. Here again, the point where a Synology’s cost / performance becomes not worth it is reached rather fast, hence the earlier suggestion to look at other solutions.
With Plex, transcoding happens on the server. If you’re playing back on a computer, or a device that’s very compatible, format-wise, you might be able to get away with a Synology (check the Plex forums to see exactly which models you’d need for your use case), but if you want to go from, say, Plex to an AppleTV or an iPad, and want subtitles, you’ll quickly be in trouble, especially if you have kids who also want to do the same at the same time. Any video processing (so the stuff that isn’t meant strictly for compatibility, think upscaling / denoising / color correction / whatever) would happen on the playback device, and, as I said earlier, that’s another can of worms and something much more suited for AVForum than the Roon Community site.
No: the NUC would be doing video and music transcoding/upscaling/room correction/whatever and video transcoding and handling two massive databases of metadata, and god knows what else.
ROCK is limited, and for good reason: it makes it predictable, it makes it stable, it makes it possible for you to go buy a cheap NUC off the shelf and have an appliance that’s supported and would otherwise have cost you a couple of grand from someone who buys an HDPLEX or a Streacom case and marks it up.
It is good the way it is, so unless you really know what you’re doing, you’re probably better off letting it do its thing.
If you’re willing to invest time, you can McGyver something that’s pretty damn integrated and very high quality, for much, much less than what you’d be looking at off-the-shelf is what can be done. Want a few thousand BluRays with full metadata information, controlled from an iPad and piped to a color-corrected 4k display device with Atmos sound and room correction ? Doable - hell, I’m sure there’s a plugin somewhere to dim the lights for you.