I'm not sure where I got this from, but what I use as action for disabling a track is a custom action, consisting of these two actions:It may be worth pointing out that there's no intrinsic "track disabled" state in Reaper as such. It's done through a series of actions on a track. You can drop this script in (needs SWS) and bind it to a key to toggle track disabling in a way that's maximally CPU efficient.
FWIW, for me, Reaper shits the bed on 4000 empty tracks (i.e. crashes). Even with a fresh profile. But nobody else seems to be complaining about that, so take it with a grain of salt. However like David, the idea of a 4000 track template is mind-breaking for me, so this problem isn't one for me in practice.
SWS: Bypass FX on selected track(s)
Track: Set all FX offline for selected tracks
Works like a charm.