> * the real problem is getting the bloody things out again after 10 years of weathering or other age effects. *

Try putting grease on thread as you drive it in. 90% or so of the bolts and coach screws i've removed from hardwood or treated pine are reusable after 15-20 yrs in Oz climate. They were all greased up by my dad going in. The ones that failed are rusted badly & the grease wasn't coated around the shaft. The good ones have signs of corrosion (we use galvanised), but essence of grease and the thread is still there.

We don't use Pozi, but use hex drivers (coach screws & bolts) or their negative female equivalent (bugles), which an allen key fits into. We also pre drill holes.