I heard dyeing the Spring Guerrilla gives better rewards?

Whoa! Sharp observation skills there. Matching attributes increases the reward slots, and higher rarity tends to extend the time and improve the quality. Your hypothesis is very close to the truth!

I also send them with UR6 Cor, Merlin, and Nina every time. It takes 14 hours, but sometimes I get diamonds.🥸

If your attributes are mixed, you get 1 type of reward. If you have 2 people with the same attribute, you get 2 types, and if you have 3 people with the same attribute, you get 3 types of rewards, so matching attributes increases the reward slots. Higher rarity increases reward quality at the cost of longer expedition times. Note that UR+ or LR+5 are treated the same as their base forms without the plus. The best setup is to send 3 people of the same attribute (LR) on an expedition. If the expedition time is 14 hours, it's considered optimal.

Excuse me. If a character is sent on an expedition, can they not go on other expeditions until they return? If so, I'll prioritize the guerrilla missions you mentioned... 🤔

This might be a bit hard to see, but it's help for the expedition. There's a faint '?' in the top right of the expedition screen, so please take a look if you can! 😊

I received some very valuable information, so I'd like to take this opportunity to thank you again and report the results of today's Spring guerrilla expedition. With 'Cordi UR+' 'Merlin SSR+' 'Nina SSR+' I sent them on an expedition and received: 'Level 5 Unidentified Rune x1' '150,000 Gold' 'Witch's Heart Shard R25'! It was more luxurious than usual! 😀

For reference Results of 3 Aizen LR