Is there any software I can put on my Debian server that would

Your question is pretty confusing.

> low spec device

What does that mean? Most devices have dedicated H.264 decoders.

> server-side decoding/processing

What decoding/processing?

You said...

> without downcoding it

You could just export the files via a web server and open the URL with a media player on your low spec device.

Plex has quality settings.

>>58475468
This is what happens when someone who doesn't understand technology and/or doesn't know the proper nomenclature, but thinks they do, asks questions... they get really confusing fast.

Also, this >>58475750

OP, Plex will only transcode to a lower quality server side if the device can't handle the original stream. The transcoding process that Plex uses to stream simultaneously is a pretty standard setup. If it transcodes, the device doesn't have the ability in the first place to play the original (either because of no dedicated H.264 decoder, or network connection speed), then there is nothing you're going to gain by trying to force it. If it's not good quality, try getting a faster device/connection (because it's clearly shit and there's no amount of tweaking to make it better).

Good luck.

>server-side decoding/processing
So you want to stream decoded raw pixel data?
Assuming it's a 1080p blu-ray movie with 4:2:2 chroma subsampling at 23.976fps you'd be looking at a data rate of ~95MB/s. Even with gigabit ethernet you may struggle to get that sort of speed consistently. If you were hoping to use wifi, forget about it
https://en.wikipedia.org/wiki/Uncompressed_video#Data_rates