Can I notch roof posts to install deck beams?
Notch the support columns? 2x6's for joists when 2x8's fit? Yeah um, stay away from those  people. Concrete would be best overall since it's impervious to most everything from water rot, to termites to air leaks.

With concrete you'd want to cinder block the perimeter, throw down 4"s of rigid foam insulation &amp; pour a proper 4" thick slab on top of the insulation. However, there's no problem with wood either &amp; that would be much more DIY-friendly.

With wood you'd do 2x8's everywhere &amp; would box-around the support beams, definitely do not notch the supports. 2" rigid insulation would go on the bottom &amp; on the perimeter frame, then fiberglass batts can fill in the rest.