In your original post, it was mentioned that you probably expect not to be able to stay in your current apartment forever.
So what is the alternative then – would such an apartment be an option to rent again? And if so, at what price until then? Do you currently have special conditions because you are a long-term tenant and haven’t had an adequate increase for a long time, and a new rental at market level will then be much more expensive?
Currently, on Immowelt, about 8 to 10 euros/m2 are being asked in Osnabrück.
So 1000 to 1200 euros cold rent is a current market price for you.
Topic "getting the money back in 15 years." Assuming you pay 500,000 including renovation, as a loan. The equity for the incidental costs and as a cash reserve during the renovation. This is not a recommended way of looking at the financing calculation but for better "moral understanding":
In 15 years, you should have paid off approximately the incidental costs plus at least (roughly) half of your loan. So you have around 250,000 from the house that then belong to you.
Can you at least sell the place for those 250,000 (i.e. with a purchase price of 500,000 in 2020 still a sale price of 250,000) you break even on the story. Just like with the rental apartment - 1200 cold * 15 years is at least 250,000 even with only minimal rent increases, which are really gone – in both cases you have lived there.
The trick: if you get more than 250,000 for the place in 15 years, that is your profit. Even tax-free. For that, you don’t have to fear an owner-use termination, no rent increase and you can renovate inside as you like without asking the landlord. And if you didn’t have a garden before, you had a garden all that time.
PS: What is your problem with the garden? Do you currently even have one that can be accessed from outside? Do you currently even have a garage? Why would you then miss it at the RH?