Private loan - what should be considered?

The tax office applies a market-standard interest rate (I believe 5.5%). Anything less in interest you pay counts as a gift. If the interest savings exceed the tax-free allowance, you have to pay gift tax. We once calculated this and I think it was something like a 100k loan interest-free = 20k gift.

That sounds good and is in the ballpark of what we had in mind. I’ve now calculated as follows: Suppose one has to borrow 600k, then at 4% interest and 2% repayment over the entire term, that would be 390,398€ in interest. If I now borrow only 500k at 4% and 100k at 1% and use the saved rate to repay the private loan, I’d come to a total saving of about 55k. Even if the inheritance case occurs for the remaining debt, the inheritance tax is lower. Or am I making a mistake here? Mathematically, this seems like a good solution, setting aside the family/emotional component :)

Also, you should consider that the uncle might realize that he could now get over 3% risk-free for a fixed deposit. He might then withdraw his offer or adjust his expectations regarding the interest rate.
I would only do it if you don't see the uncle constantly. He will scrutinize every major expense on your part, since you could have also paid off the (cheap) loan instead. Unfortunately, I've seen this happen often among friends, regardless of any prior intentions or agreements. Private individuals behave very unprofessionally when it comes to money or business.

It's a matter of personality. My wife would be strongly burdened by it, I would reply, "You lent me the money, what I do with it is my problem." And to avoid conflict when things change, you make a contract.

You can receive a tax-free gift allowance of 20,000 EUR every 10 years, exceptions are spouses, children, and grandchildren. If everyone stuck to this, the federal budget would be twice its current size and the Federal Republic would be debt-free.
What you make of this is up to each person ;)

Just the fact that the tax office applies 5.5% interest. Completely absurd... do I then also have to partially tax a private loan as a gift because I currently get 3.5% there?

First of all, everything I write is based on the assumption that everything is reported to the tax office - which I do not assume at first, as it is totally uncommon. You don't do that within the family ;-)

I calculated like this now:
Assuming one has to take out 600k, then at 4% interest and 2% repayment over the total term, that would be €390,398 interest.

If I only take out 500k at 4% and 100k at 1% and use the saved rate for repaying the private loan, I would come to a total savings of about 55k.

Then you pay too little interest and the tax office assumes a gift, since market interest rates have to be considered.

Just that the tax office applies 5.5% interest.

Yes, that's absurd. Because this regulation does not exist. The 5.5% are only applied if no interest at all is agreed upon in the private loan.
So if you get a loan today at 3.8% for 5 years from your uncle, that is market standard and the tax office approves the contract.

Oh yes, please remember for the lender that interest income MUST also be taxed. So it must be declared as capital income in the tax return. In principle, the borrower can also deduct the interest as income-related expenses (if income-related expenses are even claimed in the individual case such as for business/rental).

I would not worry about these topics at all. Draw up a contract so that the lender has some security. Lend or give out money. Repay and done. That is nobody else's business. Legal? Nope. But pragmatic and common.

First of all, everything I write is based on the assumption that everything is also reported to the tax office - which I do not assume at first, as this is totally unusual. You don't do that within the family ;-)

how do you want to transfer 100,000.--€? If that is supposed to completely bypass the tax office, careful planning is required. It is easier to date the contract to an earlier time when the interest rate was still at 1%.

As a fan of simple solutions, I would agree to send the invoices of a few trades to the nice uncle. He pays them, and that's it. So, now the overly correct ones may come... ;-)

Wanted to ask just now, because banks have to report transfers over 10k, right? But yes, the easiest is probably just to have invoices paid...