Unless we're talking about going all out, practicality and affordability be damned in the name of every marginal gain possible, it's probably the road bike I already own:
Cannondale CAAD13 (alloy frame), mechanical 105 with hydraulic disc brakes and a set of alloy DT Swiss wheels (PR 1600).
There is no use case for TempleOS. It's just a curiosity, albeit an interesting one.