I want to change my email for this account to a different email address that I check more frequently, however, it says that that email is already in use

Since you are the owner of the email that's already in use, how about you try logging into Deckbox with it? You have to do a "Forgot password" thing, but once into the unneeded account, you could either switch your inventory over to that account since it's already using the correct email, OR (probably preferred) change its email to something else or delete the account entirely.

It's possible that you may have to contact Deckbox directly, which you can do from Help.

Or you could setup emails from deckbox to forward to the correct email.

Turns out I had a second account with 0 trades and 0 cards in the binder, so I just changed emails on that account to a random spam one and deleted all my info off it.