Follow-up financing - better starting position

Hello everyone,

even if an exact answer to my question will not be possible, I would still very much appreciate an assessment.
When we financed our house, it looked as follows:

Purchase price all in all €900,000
Of which financed: €690,000
Monthly family income (1 child): €7,300
Installment: €2,500
Interest rate: 1.95%
Fixed interest period 10 years
Remaining term 5 years

In five years, a follow-up financing is due on a remaining debt of €510,000
We want to fully finance the remainder over 15 years. Currently, the family income has increased to €8,500 per month,
The standard land value has risen significantly, so the property value should be around €980,000.
My Schufa score is 99.2

Assuming the key interest rate remains as it is. What interest rate could I expect for the follow-up financing (even if this is very hypothetical)?

As of today, in my opinion, between 0.7 and 1.3%

You could already finance immediately afterwards. Some banks offer it.

You could already finance immediately afterwards. Some banks offer this

What is the maximum period during which this is offered? 5 years in advance would already feel like a lot to me.

What is the maximum period for which this is offered? 5 years in advance would already feel like a lot to me

5 years…..I have already read that… The question, of course, is what this service "costs"!

That definitely sounds very good. I just hope that there won’t be any strong upward spikes due to inflation.