I'm hand-pricing my Lumi ornaments,etc while GA-less, but I haven't been able to find them besides by searching by name for each different decoration type. Is there a category under the "Item Use" dropdown that all the Lumi decors fall under that I could be using instead? x_x

No there is not. But you can make your own categories. It will take a bit of work the first time, but search for broad stuff like ornament, garland, tree skirt, bauble, topper, etc and put them into a category you've made.

Thanks for the answer. I wonder if I should make a suggestion to add it as a shop category though, just in case that might work out. xD

Would you be able to answer if the category is set permanently for that item, so then once it sells out and you put in a new one, that you don't have to set its category again?

You'll have to move the new copies of the particular item to the category again if all the other copies have sold out. It is not permanently set.

Ahhh darn, that's mildly inconveniencing xD Thanks for letting me know.