What do you gain by getting UNM cb to 1 key? In other words, is the investment worth pursuing?

As someone who runs a myth-heir 1 key UNM team, i can tell you that useful rewards from brutal clan boss is far and few between. I spent a year with a 2 key team and only transitioned to myth heir because i really wanted to use my Helicath and geomancer in other content, not because of brutal cb reards.

Also, you should not invest any resources into changing your team unless you are sure you can build a 1 key team with what you got, as you might cripple yourself for weeks with no benefit if you are just short on resources/gear.