[JRandomFreeper]

You would know. Plants do horrible, and when you pull them up, they have nasty, fat roots.

I'm researching good nematodes that kill the bad nematodes.

/johnny

[SisterK]

I feel for you...root knot nematodes are extremely wicked. If you can afford to pasture part of your space for 3 years, you can starve the nematodes out (they don't like grass roots).
I recall some farmers having success with a very thin application of oil to the soil. Suffocates them.
I do not know if diatomaceous earth is even a consideration.

Studying different fields, I tend to think that root knot nematodes do not like heavy soil, but flourish in sandy soils. So, without intending to, you have already begun the battle by adding so much organic matter onto your sandy garden. It is unfortunate that baking the soil will also kill some of the good flora and fauna. Perhaps you can convince the grandchildren to rake away the organic stuff, then bake the sand. (Finding good nematodes seems a lot easier.)
Wish I could be more helpful. I have been blessed with super heavy soil with serious clay hard-pan...So not much practice whipping root knots these days.