Interest averaging beneficial?

To see if I can lower the mortgage costs, I'm thinking about 'interest resources'.
On various sites I come across that this is often less advantageous than it seems, which is why I wondered if I'm overlooking something.

My mortgage:
- Interest period still 15 years fixed at 3.15%
- approx. 320k remaining

We do not intend to move.

I can open the interest and fix the interest again for 20 years at 1.57%, for this I have to pay a 40k penalty or with penalty interest at 2.67%.
One-off costs (deductible) €250, saving of almost €70 gross p/m. I found a website for interest rates calculator.

At 1.57% interest I pay a total of approx. 390k over the remaining term of the mortgage, at 2.67 approx. 440k. The difference between interest means or paying the penalty is therefore a total of 10K to the disadvantage of interest means, with the advantage that I could do this immediately.
In addition, the (penalty) interest remains deductible, while I cannot fully deduct the one-off fine.

Are there other points that you should consider with interest means or is it actually just attractive is the information on certain advice sites simply incorrect?