With only 4GB you might need to go even smaller if you've got concurrent usage as I think the buffer is per stream.
I see you also removed --vfs-read-chunk-size-limit off from your mount script. This defaults to off so removing won't help - if you're still having memory problems I'd set this again to something like 256MB of 512MB, but you're going to struggle to play remux files IMHO if you don't have enough ram to buffer the stream. I don't know what size chunks my setup requests, but I have no issues.
If you're happy with only being able to play files once they've been uploaded then this is fine. However, if you have a lot of files that are pending upload and you want to Plex/Sonarr/Radarr etc to be able to see them, then this is where the unionfs and rclone_upload folders come into play - e.g. unless you make your setup a lot more complicated you can only upload 750GB/day. There are ways around this e.g. my setup allows me to upload at my full 1G line speed 247, but it took me a few hours to setup, but it means I don't have files waiting to be uploaded for more than a few mins. I still need the unionfs setup though so that Plex doesn't have to rescan thinking the file has changed when it moves, when it hasn't etc etc
Check out the new scripts on github as it accounts for this e.g. if it tries to mount and there's no internet connection then it tries again a few mins later. This check is useful if your mount ever drops as it will automatically remount.