Buy an apartment for the parents?

Nothing should be handed over free of charge here, the parents pay the €850 rent if I understand correctly. Everything else would, in my opinion, also be an unwise decision. You first have to be able to afford to possibly give away decades of residential rights without consideration. Normally, the parents should have made some provisions. Thirty times the annual cold rent is absolutely normal in popular locations, it won't get any better.

I have siblings, so a contribution from my parents towards the purchase price is out of the question for us.

They are supposed to and want to pay rent as well. I am still having some trouble with that point mentally. Of course, one could say that they might have rented the apartment anyway and paid the 850 euros. On the other hand, with their pensions (together about 2,700 €), that's quite a bit for them. If I want a reasonable repayment, I also end up with a rate of about 1,200 € on the financing calculators online. With reserves etc., that leaves quite a burden on us.

And since we do not live 20 minutes south of Bonn, directly on the state border to RLP (no daycare fees), the purchase price is unfortunately normal.

Bönnsche prices in Unkel? - yes, with our landscape you can indeed get the idea of a "Kurtaxe"

I am from the area myself, in Hennef, not far from the border to RLP either.
Currently, many young couples are financing there because there are no daycare fees... in Hennef, I already pay a lot of money for 2 children (one under 3)...

Bönnsche Preise in Unkel ? - ja, bei unserer Landschaft kann man schon auf die Idee einer "Kurtaxe" kommen

Oh, are you from Unkel?

Bonn itself is quite a different story.
At first, I searched in Königswinter and Bad Honnef, but there age-appropriate 3-room condominiums start at €400,000.