No AMD CPU/GPU. Plex does not support hardware accelerated transcoding on AMD GPUs.
The Celerons will transcode 1080p, but will struggle if they have to burn in subtitles. I’m not sure how well they will transcode & tonemap 4K HDR. They can transcode 4K using hardware acceleration. I’m just not sure of their ability to tonemap HDR to SDR in real time (I’ve never tried to do so on a Celeron).
Of those three, the i3 is your best bet.
I run Plex on a Lenovo M90q with an i5-10500T with Ubuntu 22.04. Transcoding 1080p Blu-ray rips is not a problem, including burning subtitles. It will transcode & tonemap two simultaneous 4K HDR rips. However, it cannot burn subtitles when transcoding & tonemapping even a single 4K HDR media.
On Linux based systems, burning subtitles uses the CPU, even if the video is transcoded using hardware acceleration. It is also a single threaded process, so cannot take advantage of multiple cores in a CPU.
You can use the single thread passmark rating as a rough guide with respect to subtitle burning. There are no exact numbers, but you can use other people’s experience as a guide.
For example, the J3455 in my DS918+ has a single thread rating of 807. It struggles to burn subtitles into most video.
The i5-10500T has a single thread rating of 2340. I have not stress tested it, but burning subtitles into two simultaneous 1080p movies is not a problem. However, as mentioned above, it cannot burn subtitles into a 4K HDR BD rip. It simply does not have enough horsepower.
Subtitle support varies greatly across Plex clients. Subtitle burning is not always required when transcoding.
Plex Android / Android TV clients, such as the Shield, have good subtitles support. PGS, VOBSUB, & SRT subtitles should not be a problem. ASS subtitles will be burned into the video stream if enabled. Note that Plex is working on adding ASS support to Android clients. A pre-beta forum preview is available. See this thread.
Plex Smart TV clients generally support less subtitle formats than Android clients. Currently, on most clients, enabling PGS, VOBSUB, & ASS subtitles results in a video transcode and subtitle burning. SRT subtitles can be an issue under certain conditions, such as if the audio is transcoding.
Other Plex clients - Roku, AppleTV, etc - have their own set of limitations (I don’t own those, so can’t provide first-hand specifics).
