Is it possible to separate all installed packages into which toolkit was used ?
Qt, GTK, and other(s).

I'm on Arch + KDE, and want to determine if installing KDE Neon is feasible.

Have a look at https://wiki.archlinux.org/index.php/Pacman/Rosetta

I already did.
Under the paragraph "Querying package dependencies", Arch has one notable omission at : "List all packages that require a particular package".

The closest thing I can think of is pacman -Qi on all packages and then grep for gtk3, libgtk, etc on the Depends on: and Optional Deps: lines.

Sorry, I'm not much help here, am I

You checked the Arch forum? They are somewhat no nonsense but pretty competent and always up for a challenge

No problem man. You're trying to help me. Just mentioning that I already tried that.

Later this weekend I'm going to register at the Arch forum and give it a shot there then.
Apologies for the late response.