I have a very cool Core 2 Duo laptop here that runs Linux Mint.
And it is pretty aweful. Would love to put Fedora Kinoite (Atomic KDE) on there, manual upgrades on shutdown, minimal set of apps.
But I dont know how well Plasma works on such old hardware. It is pretty bloated and messy sometimes, Dolphin and plasmashell are my biggest worries (the whole panel and widget stuff is sooo complex).
Has anyone tried Plasma?
An alternative would be LXQt with KWin once 6.1 comes out and it has full Wayland support.
I tried it on a real old vista laptop once and it ran terrible, but so did everything that wasnt XFCE. But a core 2 duo might be able to run it acceptably. I say give it a try :)