with a 40l coldbox tub already at hand it may be counterproductive to go biab.
While Biab when conducted by a skilled brewer can produce brilliant beers, its a 'cheaper' route to brewing negating the need for a tun and the investment in it.
However if you already have the tun avoiding using it so you can manhandle a hot wet bag of grain at the top of a step ladder or however you plan on post mash boil prep
may not be the best course of action.
No need to go full on 3V either simply co-opt the FV prior to final sanitation to drain the tun into while you sparge or to collect each batch sparge if you go that way
IM not trying to knock BIAB or its results, just suggesting its a solution to a problem you dont have when a 40l tun is sat ready
you may disagree that rinsing out a tun is preferable to hefting 10k or so of hot wet grain about
