There's nothing that prevents you from running say XMonad and Emacs (every buffer as a new window), Firefox (every tab as a new window), urxvt (without tabs) thus delegating all window management to XMonad.

I did try this for urxvt and it was quite efficient. No idea whether it will run well with hundreds of e.g. emacs buffers.

At least, with GridSelect [1] the selection mechanism scales well to hundreds of items.

[1] https://hackage.haskell.org/package/xmonad-contrib-0.13/docs...