There's a config option to disable mobs from dropping their armor, but none exists for other items (artifacts, shields, weapons).
An option to disable dropping items as well would be nice, thanks!

As far as I know, those are controllable through datapack - you override the mob's loot table file and replace it with whatever you deem appropriate. Mobs dropping their armor is something handled only internally, which is why we provide an option to disable it.

I've been digging through the data files and have yet to find a way to disable item drops like the shields - even when removed from the body pieces vanguards will still drop their shield no matter what. They seem to be handled in a similar manner to armor.

Interestingly their held items are classified as "boss gear", although some pieces do indeed appear to be hardcoded.

Can confirm, here is the geomancer loot table: https://github.com/Infamous-Misadventures/Dungeons-Mobs/blob/1.18/src/main/resources/data/dungeons_mobs/loot_tables/entities/geomancer.json
But it still has a low chance to drop the Geomancer Staff artifact (dungeons_mobs:geomancer_staff), which is hardcoded as a hand item here: https://github.com/Infamous-Misadventures/Dungeons-Mobs/blob/1.18/src/main/java/com/infamous/dungeons_mobs/entities/illagers/GeomancerEntity.java#L152

Indeed, from what I can tell item drops are hardcoded the same way armor is in the mod currently, an option to change it would be much appreciated!

Oops, that's my bad, I read your question as wanting to control their loot drops. I see no reason why we cannot additionally include the config options for their held items as well.

Added the config option to toggle our mobs dropping their held items.