Cache is not needed for Plex with the right settings. It seems you found the right thread to learn about these things already. Lots of good info there.
As for it being on a Pi, I'm not sure if that's a problem or not. I would think that direct-streaming should be doable.It should only be relaying the data then, which should be a fairly trivial operation. It is only a long buffering-time before playing (but smooth thereafter) or does it frequently stop to re-buffer mid-play?
The latter issue is something a lot of people face and is not (necessarily) hardware related. Is the result the same on the web-player? All I can really say is that it is perfectly possible to get this working - and without cache - but exactly what the trick is I am not so sure about. Since I don't use Plex myself yet I only know what I've learned from Animosity mostly while helping other users and my info on this is incomplete. His thread is no doubt the place to go to learn more on this.
I would however suggest that you just eliminate the hardware as an issue before you begin by monitoring the CPU-use while you play a stream. Check that it is not maxing out the CPU, as that's the only plausible bottleneck here I would think. If it is not (and I don't expect it will be without any transcoding) then it should be workable. I also believe I have seen other users run Plex on a Pi before, although I have no idea of the spesific models involved.
I'm sorry I can't give a more spesific answer as it relates to Plex, but If you have questions about spesific rclone commands and such then I will be happy to answer any questions you have about those. As I understand there aren't any special flags that should be needed for Plex-use, and as far as running on a Pi is concerned the flags you ought to pay special attention to would be anything that used a lot of RAM. Avoid unnecessarily large buffer settings (which are usually not needed anyway) and don't go overboard on --drive-chunk-size (which is for improving upload performance anyway and probably not that relevant to this issue). 64M is more than enough for good performance. Do not confuse with --vfs-read-chunk-size which is best left at default. The rest you will have to ask me more specifically about I think