First, two months is too long with no d-u, when debian bullseye is under heavy development. Better d-u at least twice per month -- every week is better.
Next, I don't know/have the GTK problem, but I would work first to resolve that. I googled
Gtk:ERROR:../../../../gtk/gtkiconhelper.c:494:ensure_surface_for_gicon:
and there are some interesting posts, including one on arch, that might help. Maybe uninstall gimp and darktable, then d-u, then reinstall if they are installable.
Worst case if nothing else works (... and I have done this, some years ago ...), let all the packages to be removed go, then d-u and get your basic system stable and with a clean set of remaining packages, i.e.:
dpkg --configure -a
apt update && apt full-upgrade
apt clean
apt autoremove
then reinstall your plasma-desktop and kde packages and anything else that you need. (This very thread can be your guide to replacing what was removed). Be careful about forcing installations, when there are dependency problems -- maybe better to download the .deb from the stable repo, if you have to have a working package and the unstable version won't install.
I hope this helps.