Let me offer a completely different suggestion.
I think the request for “parallel streaming” is unlikely to be implemented, because it’s wildly complicated, and because on many (most?) client platforms Plex doesn’t manage the TCP sessions and stream data at all, relying instead on the connectivity and video libraries provided by the vendor.
There’s something that folks running PMS on Linux can do today. Look at enabling the BBR TCP congestion control algorithm.
This is primarily a sender-side optimization, so making this change on the server can benefit all of your clients. In helping a friend, we saw significant changes for one Plex client that really needed it, and a minor improvement for one that was already working great.
It’s “somewhat” unfair to other connections. That may be exactly what you want. 
Note that Google is using this widely, including for YouTube.
Netflix did the work to make this available for FreeBSD, too. It’s in -current and expected to be in 13.
