«Well, I get your point, even with the high valuations.
But my spouse (if you have one) would like to reduce our debt. As said, I believe my margin is too high based on [reasons]. If you adjust it to market conditions, I can maybe convince her to keep the mortgage level and our other investments as is”.
I would start with asking for an adjustment. Repaying is just one argument you can use, since you can’t credibly switch with having a fixed part. Cancelling that early would indeed be prohibitively expensive.
It’s just an idea, there’s nothing to lose. Depends of course whether your current margin is too high or not.