Pookly As mentioned in the initial post: Yes, that was implemented already when this thread was created.
I just finished defining structure configurations for all the vanilla structures, including equipment tables for the various mobs to spawn there. To summarize: If there are armor trim templates to be found in a structure, those trims will show up on armor worn by the Loonium-spawned mobs in those structures. Typically the armor set worn by an individual mob will be of one material (unless vanilla generates bonus armor), but each armor piece has a 50% chance to be generated at all.
There are a few special armor and/or weapon equipment tables. For example, in strongholds any zombie or skeleton variant can randomly spawn with a full set of black-dyed leather armor, where only the helmet is trimmed with the Eye trim in amethyst material. Or in woodland mansions, where any zombie or skeleton can spawn in black leather armor without helmet, but with golden Vex trim. Structure configuration files can also force particular mobs to spawn in adult or baby form to prevent e.g. piglins in bastions from spawning as (harmless) baby mobs. Speaking of piglins, those are configured to spawn in a way that makes them aggro on players automatically, even if the player wears gold armor. (Unfortunately I have not yet found a working way to auto-aggro other neutral mobs yet, which mostly affects endermen and zombified piglins.)
I think the Loonium could use a new challenge advancement now, requiring a player to kill each possible Loonium-spawned mob, which requires setting up the flower and enough mana within many special structures, such as monuments (for guardians), villages (for zombie villagers), ruined nether portals (for zoglins), nether fortress, bastion remnant, and end city.