Unholy Harbinger: Plans for the Tomb Spyders are to be anti-tank, mybye hide them behind the monolith so they can jump out and rape them, and to tie up other units if nothing else
Monolith is gonna pull warriors out of CC, and of course, blast the heavy armour elites or heavy tanks if a good opportunity rises, If I need to fit to the situation I might deep strike the monolith behind their lines if I need to get to the weak points and just let the warriors walk out of there and blast them to death.
I like your list better to be honest, destroyers I think work better for me and Personally change those scarabs to tomb spyders, they will be making them as they march forward anyway
hardcore: Like I said when you posted the Phalanx Box, about 150 pounds to use (already bought a lord for which I used to test my colour scheme.)
I might just forget about it to be honest, but keep into the modifications though