Do the layered approach. Follow with a layer of washout/"crush and
run" on top. You don't need much, just enough to fill all the gaps
from the previous level plus a little.

If you wet it and roll it out, you can get a very solid, reasonably
smooth surface.

If you drive over this for a few years and then lay down some cement on
top of it, you will have a floor that isn't going to shift anytime in
your lifetime.