I'm not super experienced with spiderfang, but you could fit that into the superbattalion if you cut out a bunch of spider riders. I've heard 15 is the sweetspot for spider riders too
3 fungoids is probably too much, thats 270 points. You'd be better off running skragrott, ideally with the arachnocauldron and a balewind which is 300 points for a much better wizard. He generates a similar number of CP if he's your general with the added bonus of controlling the moon once per game.
Otherwise i would recommend swapping one of those fungoids for a madcap shaman. The madcap can still double cast once per game and has a spell to give a unit -1 to be hit by shooting attacks. More importantly he has access to the Moonface Mommet artifact which gives a nearby unit -1 save. Squigs at effectively rend 2 can chew through a ton of stuff.
I've run squig herds in 18/12/6 which seemed fine.
One mangler + support is probably the way to go, snufflers are amazing in squig lists so you should bring at least one unit.
For anti ranged gloomspite gets sporesplat fanatics, madcap shaman, and the malign sorcery endless spell prismatic palisade.