Seems a reasonable list, I certainly think heavy destroyers are better than standard one now they can be taken in single units. Scarabs definitely are not a better choice. They are very expensive for very little IMO so keep the heavies as they are. I'm not sure about Command barges as well, I haven't really used them but from what I hav heard they have taken a massive hit, I would say you would be better of with someone liek Szeras (for his +1 bubble on RP for the immortals) or even just a standard overlord to sit at the front and take the hits with nightmare shroud?I'm not sure about the heavy destroyers. They have good range but at 750 I feel like maybe scarabs would be a better choice. I wanted to take tomb blades since they're ridiculously good in the new codex but unfortunately they're super expensive (and no proxies allowed).
Apart from that seems good for 750pts, have lots of individual destroyers is going to be irritating to deal with.