There's no magic that really fixes if you WAN link is over saturated other than playing at lower bitrates.
The way plex works is it asks for the file. There is no concept of multi threaded or anything in Plex to do that. It asks rclone for data and rclone delivers it. It is also heavily dependent on the device playing, direct playing or transcoding and bottlenecks on the server as well.
There are quite the number of factors that go into playback.
In a perfect world, I'd probably want to have a mount with a disk based caching so I can keep files in a stage area that are being played and grab them while playback is going but it's tricky as there are a lot of factors that go into that to make it work without choking everything.
The cache backend does some of this but without a maintainer, it hasn't gotten any love in some time and needs some fixes to make it really useful for me.
You really want to limit the streams in Plex to something around your capacity. You can do that in the settings as it doesn't matter what my connection as I configure my streaming to match what the connection limitations are.
There is an old saying that you can't fit 20 lbs of stuff in a 10 lb bag so you have to work within the constraints given to you.
If a link has say 50Mb/s and it's saturated at 50Mb/s, adding more streams would make it worse since it is saturated. Are you meaning something else than 100% utilization as that's commonly what's referred to as a saturated link.
How? Plex is asking for sequential parts of a file and reading it and only reading what it's asking for. Breaking that up into bits and re-assembling it would make it far worse as it's sequential IO as you'd have to wait for that work to complete.
As I mentioned a bit earlier, it sounds like you want something like the cache backend to read ahead and keep things local if you run out of capacity but the problem still remains if a link is saturated and the player is playing more bitrate than the link can support, it would not ever catch up.
It is entirely possible that OP's ISP is throttling per-request capacity during peak use. Thus ~30Mb/s per "stream" (i.e. per request) but combined streams are closer to 100Mb/s max rate.
Not sure why whether OP meant megabit or megabyte per second is relevant, as long as it is consistent across measurements. (Although I do wish people would use Mb for megabit and MB for megabyte as designed so it would be consistent across different poster's comments. Hell, even Plex uses it incorrectly.)
More specifically to the original question, its not rclone have you (OP) experimented with cache vs. VFS? (I'm sure this will pique @Animosity022 interest given their other discussions on cache use vs VFS use.)