So, the pi is $80. That is without a case, power supply, or hard drive. Once you add these things to the Pi, you are into the $100-200 price range. At that point, you can just get an old desktop or a micro desktop. There are some youtube videos about it. I think they are maybe twice as big as a pi, but have intel processors in them.
They will be cheaper or similar in price and have better performance.
I just use an old desktop that has a 4th gen i5 and it runs significantly better than the pi4 does. Plus, I can just throw all my drives in the case and not need to worry about USB connected drives.
I use a J5040-ITX ATM.
I know it says Pentium but don't take that for granted. This chip is really just a lower binned i3 throttled enough to be passively cooled.
I run Plex as a docker-compose workload and bind mount /dev/dri which passes Intels quick sync accelerator into the container for Plex to use.
This enabled hw encoding. I also make sure I can direct stream from all of my clients. This setup can handle a few 4k streams and several 1080p streams.
I mainly use it hoard to losseless music and hard to find cartoons / movies.
I have an upgrade to a Pi Cluster planned but I don't recommend it unless you specifically want to run Pis.
Any Intel CPU 8th gen or newer with quick sync can do like 20 simultaneous 1080p transcodes. You could get a Celeron and have a powerful plex box. Look up guides for the HP 290 as a starting place.