Just ran a Kazuchi assault bonus quest, golden kazuchi (Izane kaguchi or something) gave 2258 exp. Why?

Because the gold doggo gives you tons of meseta / items to sell for meseta. If you don't want to use it, you can just keep it and exchange it along with other bonus tickets you don't want in the swap shop:

The tickets can be expired as well, and will still be exchangeable.

the only keys that give a considerable amount of exp are Tokyo key both silver and gold and Rappy fever, the rest doesn't give much exp, Magatsu gives SG both silver and gold keys and Tagami Kazuchi pretty much only gives Izane orbs that can be sold for 50k meseta.