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.
100% serious, look more into LXQT, or even KDE directly. The LXDE project supposedly is ended and QT is the new fork. But modern KDE, when everything is switched off, is lighter than almost anything else out there except for the lightest of light. Mind you: I use XFCE, but that's just cause I'm fussy. I'd use Cinnamon if it wasn't so buggy with alt tabbing in some games.