Guessing an accountant can answer this better than myself but if you are in a hurry for a response, my thoughts are below.
If the question how do you minimise your tax bill with the $30k bonus?
Prepaying your interest brings forward an expense. This results in you having to prepay your interest every year moving forward as if you don't prepay next year then you won't have any interest to expense against your income leading to a larger tax bill in that year.
However, prepaying is a good idea if you are expecting a lower or no income next year. This is because your marginal tax rate is higher this current year than next year, so it's better to bring forward the interest expesnse.
If your question is how much of the $30k you should use for prepayment? That would depend on what else you could use the $30k for. Leaving tax considerations of bringing forward the expense, you can leave it in an offset account to decrease your interest whilst giving you the flexibility to us the money when you need to on whatever you want.
Other way I read your question is how much of your loan should you fix for 1 year in order to prepay the interest. That calculation would be roughly
Loan amount to prepay = $30k / interest rate