Z15 loan and follow-up financing - is a building savings contract the best?

what would be - compared to the [Bausparer] - the cheaper or even cheapest option?

we are in our early thirties

The building savings contract is not at all unattractive for you, since the exact amount and timing are known to you and special repayments are not possible.
However, if you do take the building savings contract, I would also secure the entire €150,000.
With 40% savings, this means, for example, a €367/month installment at Fuchs during the saving phase and a €750 loan installment.
You can also start with €300 and later switch to €400.
Just always keep an eye on the valuation number!!!
The advantage of the building savings contract is probably the advance payment from the employer and you are definitely secured against the interest rate.

Whether this is necessarily the cheapest option, no one can predict.
If interest rates rise again, this would of course also be associated with better interest on the credit balance.
In general, inflation and salaries also tend to rise then. Since the annuity of your loan remains constant, you should actually be able to save more effectively then. That means you would probably have considerably more available after 14 years than with the building savings contract, since it has a constant interest rate on the credit balance.
The higher interest on the loan extension then applies only to a lower loan amount. It depends on the assumptions made about the interest rate whether the building savings contract or annuity loan is cheaper.
If interest rates do not rise and inflation remains close to zero, then you are not worse off than now.
In this respect, is right that you are more flexible without a building savings contract and possibly also cheaper.
You just have to make sure to get a decent interest rate on the balance and really consistently set the money aside.

So the case is described here.

As already mentioned in the other topic, forward makes no sense here.

Basically, it depends on what you want: low costs, high security that you can continue to finance it afterwards, or high flexibility.

You can already see from the statements of others that you can always chase the perfect thing, but you will never achieve it. There is no all-in-one solution. You have to decide on one of the three points above and optimize the financing accordingly.

And only then can you say what your best way would be.

thank you very much for your answers!

why would you, if using a home savings contract, secure the entire remaining loan amount?

from the feeling, I would have somehow split it now.
e.g. 70K through the home savings contract (savings rate €169, repayment rate €560 with LandesBS),
from the remaining 80K then about (rather more, since calculated without interest) 41K saved up (this also includes that I can work again after 10 years and we could save more in the last 5 years);
so the remaining about 39K we would then just repay by luck with the usual interest rate via a normal loan.

that way, on the one hand, we would have interest rate security for a large portion, but could also benefit from potential rising credit interest rates.
or is there a hitch somewhere?

for me, the above-mentioned model would almost be the jack of all trades—or do you see a flawed idea that would invalidate my whole construct?

thank you already for your help and your thinking along!!