This may not fit the 90% same machine situation but:
A few years ago, I looked around for IPC that would reliably scale to more than 1000 connections doing about 1 message/second with a few hundred bytes/message, and with forward secrecy.
The only thing that still fits that is #UDP (not TCP) over #wireguard.
For #rustlang udp means: do it yourself, no existing crates, allow for missing data, and #serde for the udp packet contents.