What is the best (longest lasting) product to use for fixing the the
non-slated areas?



I've had really good luck with a product with a name like 'Akona' or
'Acona'; it's a two-component system that gets hotter than blazes while
it cures. I fixed a broken corner of a concrete porch with this stuff
about 18 years ago, and it's still doing fine. it was so long ago that
I don't recall seeing this stuff recently -- but I haven't been looking,
either.

If the concrete being repaired isn't in good shape, of course, the
repair material will just break away like everything else, but you knew
that.