First thing: it’s likely that nothing is flat in part because of your threaded inserts. You should have milled a little recess so that the heads wouldn’t protrude off the board. I suggest you to fix this, it will not only help with flatness, but also with vibrations and noise.
You’ll have to use some surfacing bit, as you already guessed it. As the others suggested already, you have to respect the maximum rpm your bit is designed for, otherwise it will be likely to explode in your face because of the centrifugal forces. It’s unlikely, to happen, but if it does it will be nasty.
As for which one to use, I can’t really say, it depends of what kind of power your router has, I’m not familiar with those dewalts, so I’ll let the experts tell you.
To surface the board, I suggest you to find some kind of pencil, randomly write all around the board. Then try to find the highest spot on the whole surface and do a 0.5mm pass first. don’t go deeper the first time because you cannot really trust that you actually found the real high spot, and you don’t want to suddenly dig 2mm of material with such high diameter bits.
After the first pass, you’ll still be able to see the low spots where the pencil marks remain. Do another pass, this time 1mm, you’ll be guaranteed that the cutter won’t ever face anything more than this now.
Repeat the operation as long as you still can see pencils marks on the surface.
Before all that, you’ll need to make sure your router is dead square to the surface, which will not be a piece of cake if your surface is crooked to begin with. So I suggest you to check for perpendicularity between each pass and adjust. That is, if you want a really perfect, spot on result.
Yes, it will probably take you a good part of your day. No magic here, unfortunately, but it will pay back later.