@VI-Lycan-Lupgarou Ahh, gotcha, unfortunately luminmechs only spawn in certain activities but I'm glad you got that folder completed!
As you noticed, an easy way to make progress is through completing a UQ with those mechs and the ones to keep an eye out for are "Armada Training: Final Lament" and "Armada of Annihilation: Interception". As Jamesmor said, you can also find luminmechs on certain floors during Divide Quests and they are also a rare spawn during emergencies quests/trials if you run Risk Exploration missions (not recommended as they don't spawn often and give very little progress). However, your best bet during a non-UQ would probably be the Ship Infiltration Ultimate Quest as all of the enemies are mechs and you can run it solo. I should note that I've never actually tried to complete a folder there so I can't say for sure whether those mechs will count (worth a try though!). Oh, and if the limited quest is still going on you might be able to try there too.