Where to begin?
Concrete floor.......not flat. I flatened it. Oh yeah, i moisture tested 10 different
squares in the 240 sq foot room. Came back all good. Next i bought a 5 gallon of Mappei 990 urethane and 3/4 outdoor ply and a ton of tapcons. Troweled mappei, however, i switched from 1/4 trowel to 1/8 and it made the ply sheets, uneven.(very minimal, but uneven) Figured I would make it up with the 15lb roofing felt. Instead of multi layering the felt i cut a bunch down to use in a shim like manner. I started rolling the random width 4", 6" 8" 3/4 oak out and i dont think my shims are helping. Now you can see the boards leaning due to the shims. Should i pull up the boards, (3 rows worth, learning experience) and just give another overall layer or two of felt? The flooring is used and some boards are thinner than others. Do you make this up with felt paper, or am i screwed, or some secret method i dont know?
If this is confusing, i havent slept much the last couple of weeks.

Can you sand down the uneven areas, instead of shimming?

I always do the subfloor @ a 45º to the installation, so no seams fall in line with my rows.