The almost zero time invest for security updates on Debian is the main reason why I switched back to it from Gentoo. While Gentoo has stable package versions it is still rolling and I have to invest time with etc-update from time to time.
On the other hand, it was easier to get other package versions outside the repo into Gentoo without using flatpak; especially very old versions.
I like flatpak, too, but I only use it for less mature apps, I want to try and have a rapid relaese cycle I don't want to miss. Btw, on Gentoo I needed flatpak as well sometimes, especially for Qt applications; because I did not want to compile stuff I rarely use.
So yes, I find that Debian stable is very stable.
I will try out Fedora Silverblue on a seperate machine, anyway. But my main productive computer remains on Debian stable. [=