When we take the mortgage out we have an option to pay interest only for a certain number of years and then pay off our capital amount and that is interest only mortgages.
If we pay capital plus interest then it will be classed as repayment mortgages.

But in interest only mortgages you pay interest first and whenever you are ready then pay capital. So the more you wait to pay your initial borrowing the more interest you pay. Whilst in repayment mortage you pay interest and capital at the same time so basically you are reducing the borrowing amount every month

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From a jurisprudential point of view, it is an interest-based loan, and there is no difference between the two cases.
Yes, you can get rid of the legal problem by taking the money in the name of saving it from the hands of a non-Muslim, not under the title of a loan, then it does not harm you that he will take it back from you by force, since you cannot default.
Of course, choosing the second case would be better.