> It's too bad how PKHeX's Encounter Database is very limited and doesn't allow users to specify the IVs and natures before generating.
You're wrong. That's the entire purpose of the EncounterDb setting UseTabsAsCriteria
PKHeX checks at most 50,000 different attempts before giving up, so as to not hang the program for impossible requests. Load a Roamer encounter, set all the IVs to 31, make it shiny, then regenerate. You'll get a legal roamer that you can set to level 100 and hyper train accordingly to get your 6IV result.