Kirsten, about 35 years ago I met a university professor, a biologist who was a behavior specialist. I happened to sit at his table at a dinner and began to talk cats with him. He said that he had indoor kitties. I asked him then whether that was ok because I was not sure about it. He said definitely yes and he explained to me how the role of cats and their living together with humans has changed since they were living in the barn as farmhands taking care of the mice. He said he played with his kitties several hours per day and that if they were not bored he can see nothing that speaks against it.
So, for one expert opinion you will always find another.
And remember: the average age of outdoor kitties in Germany is 2 years- and now look at the age of your kitties.
It is true, sometimes they stress each other. But they will find plenty of stress in outdoor life to, because they will have to share the territory with plenty of other cats, some of them not neutered and therefore more territorial.
So you read some more books and be happy again