Is RAM usually the cause of Plex buffer bloat during high-bitrate 4K playback?

Not really. Plex buffer bloat during high-bitrate 4K is almost always a transcoding or network bottleneck, not a RAM shortage. 8GB is plenty for Unraid + Plex + a handful of Docker containers, as long as you’re not trying to run multiple 4K transcodes at once. Your CPU or iGPU (or lack thereof) is the real limiter here, and if you’re direct-playing, the bottleneck is your network or client device.