I'd say definitely keep the Spore Cysts on the Carnifex, totally worth it to protect them as they'll sit around for a turn or two eating bullets.
Add Acid Maws to the Stealers too, it's free and will help against armour when needed (just like talons help versus low armour).
Unless you really want to use the Zoanthrope stratagem, I'd suggest to get two unit of 4 instead of 3 of 3. Make them more survivable and give them some bonus while they stay at 4 models. With all the Stealers and Carnifex around, they might very well stay at 4 models for a while! To complete the Vanguards, you can add a Lictor (that'll be 5pts more in total, which you have to spare without changing your current list

). Lictor might not be that great, but I still find them fun to use!
And if possible I would also try to increase the Venomthrope size to at least 4. They are rather weak against shooting so that can help them survive some more. I would drop a few Stealers to make room for it, either 2 units down to 18, or one to 16.
For Hive Fleet you could go Kraken to support the Stealers (but doesn't really help the rest), and take the Kraken relic for your Warlord. I also like Leviathan personally as it can add some nice survivability to even small models like Stealers.
Ymgarl Factor/Maw-Claw and Norn Crown are the relics you could take otherwise, depending on your warlord.
Adaptive Biology would be my go-to Trait in that list or the fleet specific one (with Leviathan, a free re-roll per turn on your 3++ save for your Neurothrope for example could very well be welcome!).