I'm pretty sure the desktop environment has a bigger impact on performance than the distro itself. LXDE and LXQT come to mind.
Right here. About anything will work fairly well but the window manager will have the biggest impact. I like XFCE and LXDE for lightweight managers.
XFCE isn't so light, any more. Mate, XFCE, and even KDE (when adjusted) are all very close to each other in memory usage now. LXDE and LXQT are what you want.
I've not used xfce for a while now. It is sad to hear it has gotten bloated. It's low resource usage on older hardware was something I really liked about it. I guess something like LXDE would be my go-to now for something older. Maybe ill try that out on some old gear just to get a idea on the difference.