Based on my own research, it seems like each location
has specific drop lists, which all enemies there use unless they overwrite it with their specified drop, such as Watchdog dropping Fire Primary, or Brainsucker Flat Arcane Secondary.
However, like you discovered with farming Watchers, any effect can drop anywhere, it's just really rare. So you could get Flat Fire or Beast Atk Up in Pthumeru if you're lucky, but it will be really, really rare.This post
contains a list of the most common
among the primary/secondary bonuses I've farmed from various enemies so far. It may not be exhaustive.
I also have two glyphs I'd like to share with you guys that are among the easiest and fastest Madman farms I've come across:
qjpktusu Pthumeru Ihyll Layer 1, in the first bonus room with his back turned to the door
9hdhpcqv Lower Loran Layer 1, directly after the main area lamp, you can bait him up the ladder for an easy Charge Attack and maybe follow-up.
Both of these are red-aura, and the sort with the corpse (my vote for the easiest to fight).
I'm not sure if these ones in particular are more likely to drop Poison (due to their weapon properties), and I have yet to find a good farmable glyph for one with Flaming weapons AND red aura to confirm if it drops Fire gems more often like the regular Pthumerian fellows.
Please let me know if you find a glyph for Pthumeru with a madman with flaming weapons...