With SBSA my debit-amount stays more or less the same and interest portion deducts after putting in a lump sum last year. I think interest will go up by 2% by end of the year and it would be not wise to shorten loan-term and rather keep putting more money in.
Not sure what you mean here, if your payment is in excess there's only one of 2 possible outcomes. Either your minimum payment declines or your term does, if you've fixed the debit amount then the term *must* go down, there's no other choice. Anyway, the game isn't to lower the term length, it's the amount owed.
If you're quite early into your loan, most of the repayments go towards paying interest anyway so unless your repayment is a whopper you won't see much of a change in your minimum repayment amount. Is that what you mean by more-or-less the same?
I stand to be corrected, but for me, the point would be to shovel as much money into the loan as possible before any interest rate spike to gain the maximum effect. Any amount above the interest portion is applied to the capital in any event.