The best setup is and always will be to just not use landscapes.
Everythign else does check out.
By design, the physical materials must be accessible in bluprint (where you do things like issue what sound to play for them).
However - this is always done on a “per request” basis.
PcG will line trace and test the instance to make sure it can spawn for instance.
As such, this method is far inferior to what the OP was trying to get done - at least when it comes to compute time.
But since it makes it possible at all, I suppose it is the only viable way…