I think the problem may be undervolting too much.One thing to seemingly keep in mind is..while the old way seemed to be to undervolt a bit so it would clock higher without hitting the FIT voltage, undervolting seems to do very weird things on Zen2. Everyone is reporting that an undervolt actually got them consistently higher clocks, but when actually benching the machines, the scores go way down. Games too.
One thread on reddit: https://www.reddit.com/r/Amd/comments/cdkbkk/psa_undervolting_does_not_retain_performance_with/
EDIT:I see you have already addressed the downsides of undervolting. NVM
So it seems while undervolting may help clocks, it isn't the answer for actual overclocking.
As far as the 'solution' I'd still be curious to hear it even in a PM. I work in electrical engineering and can get my hands on a bit of equipment I mostly work in RF, but still.