You will get the same version of Plasma and probably older versions of some other packages you use. With 5.27 being the last significant update until Plasma 6, you’re good on that for a while. Nothing will be newer than what you already have. None of your packages will get any updates beyond bug fixes and security updates, but it will be rock solid.

Kubuntu is also reliable and has newer packages. Personally, I don’t think there’s much to gain here unless you’re set on something that doesn’t have snap and don’t mind running older versions of most packages.

Why not?

Install Debian, get a stable base on KDE Plasma 5.27 LTS while KDE 6 sorts itself out, and for everything that you need up to date (browsers, office, etc) just install the Flatpak version from Flathub.

Others have mentioned the shortcomings already, but the real question is why do you want to switch to debian? If there's a particular reason you wanna switch go for it, maybe dual boot first to see how it goes.

I'm also running Kubuntu, but I'll switch to EndeavourOS. I wouldn't pick Debian because I like having the latest versions of things as they're released, some things even in beta, but other than that I think it's a solid choice if it fits your needs.