Ok I've got every single bit of math using two decimal places, but the deposit still isn't balancing. How do you "balance" a deposit anyway?

I would recommend you perform the exact same actions that is in your code, but do it in the UI. There could be another required field or a field value that gets sourced in, like an account number.

so the "Transaction is not in balance" error might mean the GL impact doesn't balance, or something like that?