You need either integrated Intel graphics or a low end external GPU. Make sure that the NAS has one of those options.

However, I’m not sure of the NAS software will support accessing GPU. You might want to look into building something a little more custom. I am not much of a NAS guy so I can’t really say what the best NAS for Jellyfin is.

live transcoding (MKV 480p -> MP4

Both MKV and MP4 are just container formats. The container contains an video stream and some audio and subtitle streams which are each encoded using a specific codec.

Converting from one container format to the other is very cheap, it’s the conversion between video codecs that requires a lot of resources. So converting from MKV to MP4 is pretty cheap if you keep the video stream as is.

Personally, I have never bothered with live transcoding. My videos are all encoded as H.262 or H.264 and stored in either an MKV or MP4 container. Direct Play works just fine. Without live transcoding, a >10 years old Intel Celeron is more than enough to run a jellyfin server.

I’m hosting on a Synology, but not transcoding.

It’s likely more affordable to host a second jellyfin server on a desktop that is used for transcoding vs getting a NAS with the hardware.

As also mentioned you may not need to transcode unless you want to down mix to reduce bandwidth when not home. For those cases I would recommend you use handbrake and have multiple versions of the content at different resolutions/codecs as needed. Yeah it’s work.

The majority of issue I have when running locally is audio codec compatibility. So I use ffmpeg to reencode and remux my mkvs with the new audio stream. (Typically eAC3)