Mortgage repayment calculator (vanilla HTML, CSS, JS)

What are you most proud of, and what would you do differently next time?

My challenge solution is not perfect at all. Since it didn't have instructions about the calculations, i just let hardcoded values, since what i most wanted was just layout practice (however there is js to handle classes changes and error states). I need to plan ahead how i'm going to handle the mobile part, since i always start with the desktop layout.