Aha; it seems that #PLEX supports third-party #HDR tone mapping to #SDR online in the #PlexPass.
So the issue was probably not that #PLEX doesn't support tone mapping, but that #Roku doesn't support tone mapping natively. #AndroidTV / #GoogleTV must support it natively, so it did it on its own once it received the video content.
Now, I'm still going to manually tone map all my #4K #HDR10 content back to #SDR, it's probably best for compatibility's sake that I not have files in #HDR in case I do something like try to play the files locally on something that doesn't support the tone mapping, or switch to #Jellyfin or something
Fun fact: one of the main reasons I am getting #PlexPass is because of the #PLEX proxy; I will be moving out of my parents' home eventually, and the #PLEX proxy only lets you stream in SD. I can't guarantee in whatever apartment building I end up that I'll be able to do port forwarding, for a proper direct stream setup like I have now
Unfortunately though, I will still need to find my own solution for my accompanying #SFTP server, I use that to upload movies and shows to the #PLEX server