Heres my suggestion as an actual guitar tech.
Remove all shims.
Install all six strings and tune to pitch for full tension to set relief.
Adjust truss rod to get some relief, something like .005-.008 for starters.
You should then have very high action and need to lower the saddles.
I have to point out again that Warmoth truss rods (unless they've changed their minds) DO NOT provide the ability to add relief. You can only add more backbow to counteract string tension. So if you've got backbow... you've got backbow.
If string tension with all 6 strings isn't enough to get the neck to at least a straight position, then contact Warmoth. You shouldn't be having this issue with a neck AND body they created. Something is wrong.