I might be tempted to lose the vindy/defiler merge the marines into one squad (20) and buy some guard allies and griffon mortars (3?) I like the idea that chaos would have traitor guard bring the big guns
a little something like this:
Chaos Sorcerer Terminator Armour Level 3 spell familiar 150pts
10 Cultists flamer 55pts
20
CSM 2 melta gun 290pts
Helbrute
tl lascannon 130pts
Allied “traitor” guard:
Hq: company command squad 50pts
Troops: vet squad 70pts
Heavy support: 3 griffon mortars 225pts
970pts
A few special weapons on the guard maybe?
or something for the marines?
id try and keep the hellbrute cheap but if its working for you then by all means keep it
hope this was at least slightly helpful