load persistent level, load sublevel and add NavMesh to sublevel (setup it as u like). Go editor preferences - search “Nav” - scroll down and uncheck “Update navigation automatically” - save all (important).
Done, now u have usual NavMesh field, that will load and unload with sublevel, without unnecessary generation.
If u want to regenerate it - load level with NavMeshes - check "Update navigation automatically - save all - after regeneration uncheck it back and save all again.