Tarrok wrote:I have 5 Lychguard, 6 Destroyers and a Triarch stalker, but (provided I understand the rules correctly) I can't just include them in the list without bringing an entire auxiliary detachment.
Correct! You can include the Lychguard because they can be taken in the Reclamation Legion, but to field the Destroyers you need a Destroyer Cult (1x Destroyer Lord, 3x units of Destroyers) and to field the Triarch Stalker you need a Judicator Battalion (2x units of Triarch Praetorians, 1x unit of Triarch Stalkers).
Tarrok wrote:Is there any way I could upgrade the 750 points list above to 1000 points (without just adding more wraiths/warriors/tomb blades/etc)?
The Decurion is a little restrictive at 750-1250 points. You can field one, but not a good one. That's mostly because the Auxiliary formations are a bit pricey. I would say the sweet spot for a good Decurion is 1500 points and up. So, for now, I would run a Combined Arms if I was you. Don't underestimate the power of Objective Secured Necrons. I would also put a Phase Shifter on the Overlord for challenges. I hope this helps!