Tom Orotere

You couldhire a concrete floor grinding machine they have a big disc and you walk behind so no bending and makes things nice and flat and vacuum to make it smooth will take most of a day then use an acrylic sealer.
Probably the cheapest and quickest way.
Even with the vacuum attached there will be fine dust in the air which is best not to breath.

There's lots of info around in building trusses  usually  we use gang ail plates on the connections rather than long screws much stringer than long screws.
Plywood or timber cleats will also work.
Plywood shell beams also work well  this link will give you some engineered tables
https://img.bretts.com.au/long_span_beams_span_tables.pdf

As above metal oofing will condensate  tar paper is fine. Our house is a similar size very different climate  no snow maybe a couple of frosts  a bit higher off the ground is nice when you want to go underneath to move or add a power point.
We clad ours in sugi weatherboard burnt and oiled with tung oil have a few practices some I didn't burn enough and the wasps love to eat it. Any uv or water will destroy tar paper in quick order.
What rafter spacing  are you planing  1 in battens won't span very far

Sounds like the flattening will only need to be pretty
minimal Its just a surface to fix boards to. A couple of runs with a electric planer. No reason that should cause issues. What are you going to use for flooring  4ft spans are going to need sturdy planking.