You could get married and change the tax classes. then you would probably have quite a bit more net income. The money could certainly be saved and additionally used for repayment.
The tax classes have nothing to do with splitting (tax advantage). It makes more sense to stay in 4/4 before you end up having to pay additional taxes in the worst case. Better to get something back then.
Today you can get a decent policy with about €3,000 monthly payment from a reputable provider if you are young and healthy or do not work in a high-risk profession. This might have been the case with the OP.
I can confirm that. I have just under €3,000 monthly with around €900 per year (fee contract). However, I do not belong to a risk group and started in my early 30s.