It's worth remembering again why we don't do this. Even if you fully trust whoever is running the MITM server, being that it's MITMing all of your comms, and it's on a server so it's actually a lot of people's comms, it's a massively tempting target for all sorts of bad actors. Plenty of governments and criminal gangs would just love to compromise that for all sorts of reasons. Odds are one of them will eventually.

For all you know, someone else on the server is a high-value target for somebody, and if they compromise the server to get their comms, they may not be very picky about what happens to the private communications of everyone else on that server.