In the upgrade from 5 to 6 some files moved between packages and that’s not a good situation because when installing parts of the system (which happens during any upgrade - some parts are updated while other parts haven’t yet), you will get into a situation where you need to install a new package that has a file from a different older package that hasn’t updated yet - and as you can see, Apt hates that.
During Plasma 6 development, the Neon developers suggested setting Apt to automatically allow overwriting files from another package, but at this point it should have been stable and so it doesn’t make sense to allow this globally.
For now - to move forward - you should run apt --fix-broken -o Dpkg::Options::="--force-overwrite" install to get the conflicting packages to install, then do apt full-upgrade -o Dpkg::Options::="--force-overwrite" to update everything to the current state, then do apt autoremove --purge to delete the unneeded old packages.
Then when the dust settles down - see if you can get a working system, if not - ping here.