The working preset from Media Encoder is H.264 - Match Source, High Bitrate.
I decided to bite and try out exactly what the post you shared said, with Handbrake, and this result also got decimated by YouTube (worse than stock VEGAS export)
Seems like the only thing that works is whatever Media Encoder is doing behind the scenes. This is the output settings as described in Media Encoder:
3840x2160 (1.0), 59.94 fps, Progressive, Hardware Encoding, VBR, 1 pass, Target 10.00 Mbps
I've tried mimicking this in the other programs but it's just not enough information. Granted I don't know what the (1.0) is.
I keep trying with any combination of settings and it always comes out worse than stock VEGAS. I'm at a loss.