if you’re looking for an original distro, you should try void. it’s super lightweight. i used to keep away from gentoo because it was a source only distro, i would otherwise go fulltime on it, but now that it also has binary compatibility you should check that out, too.
as for wm, i love wayfire as a floating wm, and sway as a tiling wm.
i’ve had a t420 for years and it’s still going strong