Just a guess, but I suspect you won’t use many of those inserts in the back. The “active” area of the table is in the center front, the back 20% or so is unreachable by the router, as are a good few inches on each side. I’m sure you’ll use -some- of them for clamping, but probably not nearly as many as you think.
If you really want the table totally level, surface it with with the router. Then you’ve got a little better support than just the paper under the end, and it’s known flat everywhere reachable. Here’s how I did it.