What veggies give the best results in a mirepoix

This is not the answer you are directly looking for, but may be the trick

Some local Phở brewers soak the onion skins in alcohol (rice cooking wine) to extract a stock flavouring (I suspect this is Quercetin?). The resulting dark liquor is added to the broth when the meat is added

This certainly kicks up the broth flavour, a bit like MSG does in Chinese cooking (sort of)

Worth a try...

I don't think there is a magic order or timing to making Phở, as long as the flavours are extracted, and not destroyed, it comes out OK

There is no simple conversion, as you are looking for outcomes.

Typically what can be done in a pressure cooker for stock or braise type applications in 15-30 minutes might take 3 hours on stove top. An hour and a half in a pressure cokoer might be analogous to a full day (call it eight to ten hours) of more traditional cooking. Doing so in an oven (300 F, 180 C) is even better than a stove top, as there is less risk of scorching.

A good quality recipe will give you a test to know when an item is done. For example, meat falling off the bones in a braise. Stock is one of the few things where its hard to give a specific test, but you should at least have good flavor development, and thinner bones will become flexible.

90 hours is simply unreasonable for almost any type of cooking whatsoever. Perhaps they meant 9 hours, which is in the ballpark.

I suggest finding a better source recipe; if this recipe is far off on cooking time, how many other problems does it have?