I experimented with Unreal Engine 5 and the Metahuman plugin to bring a custom game-ready mesh into metahuman, so that I can export it back out of metahuman at a heavily improved level of detail, but still as a game-ready mesh. The results were unexpectedly good, and I will work on making this part of my workflow for realistic portraits.
The model, based on a real person, was sculpted and textured from scratch by myself, before placing it into metahuman.