At this time I know exactly nothing about the BM1398 protocol and cannot advise.

Remember any intermediary steps will have their own power losses, and efficiency percentages cascade. Two 90% stages in series still results in a 19% power loss, and if you 20V into 5V you still have the problem of trying to get reasonable efficiency out of an 8% duty cycle switcher. I'd be more likely to take 20V down directly with a half-bridge forward converter and custom wound transformer especially if it was a one-off. That's how a lot of your good active-PFC supplies handle bucking 350+VDC down to 12VDC at a hundred or more amps.

The eleven projects can largely be subdivided into a few project families with a decent amount of overlap. Eventually I'll be fully excised from the assembly line and can focus more on R&D. That's the dream anyway.