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Once my Chapterhouse order arrives I'll be trying this out:

Everything except the 'stealers and Doom starts on the table. The Trygon and Swarmlord push forward toward the most suitable target - down one flank if I deploy last - with the aim of forcing my opponent to handle fifteen T6 wounds with regen (and eventually Leech Essence) before they hit him on turn 2-3. Their goal is to get into melee as soon as possible so they can no longer be shot at.
The Carnifexes will cover the Tervigon et all, moving over the board while spawning a screen of termagants. Hive guard will break off from this group at a suitable location that lets them cover the centre of the table. The Tervigon+fexes can hold the most at-risk objective, while spawned termagants can scuttle off to easier ones and go to ground.

Using the reserve bonuses from the Swarmlord, the 'stealers outflank and appear on the most effective table edge. The Doom will drop slightly sooner in the most suitable position - most likely to counter an attempt to take an objective.
The majority of the army is either T6 multi-wound, expendable, or fast enough to make it into melee and minimise being shot at for extended periods. Catalyst can help to shore-up at-risk units, and there's plenty of Synapse around.

Is it worth freeing up some points somewhere to take Onslaught and potentially extend the range of my Zoans / HGuard for a turn? Am I making any terrible mistakes here?

HQ

Swarmlord
+ 2x Tyrant Guard with Lash Whips
410 pts

Troops
Termagant Brood with Toxin Sacs
60 pts

Tervigon
Catalyst, Toxin Sacs, Cluster Spines
185 pts

Genestealers with Toxin Sacs x 10
170 pts

Genestealers with Toxin Sacs x 10
170 pts

Elite
Doom + Spore
130 pts

Zoanthrope x 2
120 pts

Hive Guard x 2
100 pts

Heavy
Trygon Prime
Adrenal Glands + Regen
275 pts

Carnifex Brood
Carnifex with 2 x TL Brainleech Devourers
Carnifex with 2 x TL Brainleech Devourers
(yep, thats 24 twin-linked S6 shots!)
380 pts

I'd drop the toxin on the gants as they get that from the tervigon. Give him adrenal and onslaught as well.

Regen on the trygon is expensive.

Another spore for the zoeys as well?

Don't run fexes as one brood - you may want to split targets with that shooting.

These are attempts at 2000 nid swarms I am to blame for, in their respective threads you will find interesting ideas and pieces of advice from other dakkaites, so rather than posting those ideas myself, better refer you to their original contexts:
http://www.dakkadakka.com/dakkaforum/posts/list/321294.page
http://www.dakkadakka.com/dakkaforum/posts/list/325114.page

These two were just for fun, but then again this is a game, so way to go, methinks.
http://www.dakkadakka.com/dakkaforum/posts/list/331449.page
http://www.dakkadakka.com/dakkaforum/posts/list/342719.page

This has 150 points worth of spearhead formations, but you can always put them to better use:
http://www.dakkadakka.com/dakkaforum/posts/list/332769.page

Same for 3000 points:
http://www.dakkadakka.com/dakkaforum/posts/list/342719.page

Swarmlord only needs a single Tyrant Guard as easier to get cover with.

Why you paying for 60 Termagants with toxin sacs when you can get them for free? Madness. Reduce them down to 10 minus the toxin sacs; your Tervigon will give them for free.

Zoanthropes on foot will more than likely die before they reach the target. Give them a spore or better yet replace them for Hive Guard.

The single Trygon is just begging to eat plasma, they should come in pairs.