Here's my take on what could be done. FwIw I have 28 years in construction, including building 75+ pole buildings over my years thus far. These include simple uninsulated drive sheds, to fully insulated gorgeous shops, and some may have even been used for living quarters after we left??
Your floor around the posts. I would suggest pour your floor around the post and do nothing. Your saying the posts appear very solid and that is all you need. The rot everyone speaks of is generally caused from a repetitive freeze thaw cycle. The post wicks the moisture from the soil and the moisture exits above grade. The typical rot zone is 4" below grade. This is where the moisture level changes from wet to drying and when the freeze cycle enters the situation it becomes prime for rot to start. A pole thats set in a wet location (wet heavy soil) is a scenario that's not going to work out very often for very long.
The soil your showing in your pictures is a GREAT scenario for pole longevity imo.
Insulation: my suggestion for your attic would be install 6mil vapor barrier to the underside of bottom chord, apply your ceiling finish (metal, wood, drywall, velvet etc) then blow your attic with your required r value with CELLULOSE.
Walls: in the perfect world your building would have been wrapped with a air barrier, then insulate, then vapor barrier, then interior finish.
Since your exterior cladding is already installed and I'm guessing your not interested in removing it then I would propose this idea if budget allows...
A flash and fill.....strap between the posts with 2x6 on the horizontal plane, spray 2" closed cell spray foam directly to the backside of your wood cladding , then get the same CELLULOSE installer to blow the remainder of the cavity with dense packed cellulose, then your interior finish.
Roof...leave the sheet metal roof alone and address the flashing problem. A sheet metal roof is perfect imo, and surely no good reason to remove it to add shingles or anything else. What you have is low cost replacement and extremely easy to maintain roof. If installed correctly you should have zero issues until the paint deteriorates
if there is not any ventilation currently in the roof I would suggest installing a continuous ridge vent with soffit ventilation as well. If the current soffits are finished and unvented, you could remove and replace them with a vented version.
All of this is assuming you have a standard roof truss system, im now thinking you said gambrel roof and if thats the case then there is a entire different scenario for that....
Maybe post a picture of the outside of your building so we can get a look at what your dealing with.