In order to use hardware transcoding in Plex, you need to have a Plex Pass on your account.
That’s why you didn’t see a difference when you added the GPU. It might be that this GPU is also far too old to support HEVC.
This CPU is of the “Sandy Bridge” generation. Therefore its QuickSync unit doesn’t even have support for HEVC at all.
You will need a more modern Intel CPU. Which most likely requires you to replace the mainboard, and the RAM. If you’re unlucky, the PSU as well.
You might want to think about getting a nVidia Shield Pro. It can play almost all those modern 4K files directly. (and also pass-through the modern surround sound formats to your AVR)
For all the other Plex client types, you rather want to rip the regular bluray (i.e. not the UHD variety).