The problem is not the bandwidth/throughput. The problem is that some mediaplayers (apparently including Plex in some cases) have a nasty habit of opening and closing the file it is reading frequently. This is a quite slow operation on most cloud remotes even though it is trivial on a local disk - leading to a situation where the player runs out of buffer because it's spending most of it's time opening and closing the same file instead of actually fetching data. Well known issue in general.
The good news is we have plenty of users who use Plex with rclone, so it is doable. I just don't use Plex myself (yet) so I don't know all the specifics. One other person I helped mentioned Plex had 2 different ways to play and that one of them worked fine? Does that tell you anything? I can't recall the exact details. I'll ask him tomorrow if this hasn't been solved by then.
Otherwise, take a look at Animosity's thread were he details a lot of this stuff. He runs Plex and has no issue - cache backend isn't even require with the right setup:
If all else fails you can gently nudge him for some help, but most likely the answers you need are in that topic and thread. Keep in mind he gets a lot of these same questions about Plex asked repeatedly (and a LOT of them will be in that thread), so please do your due diligence in researching before asking him to donate his time. He has the forums to moderate after all 