Can you help me improve my build ? (Poison artillery ballista Pathfinder)

https://pobb.in/d0dN9rwBTR06

Hello guys

So I've been trying to put this together as a simple and cheap league starter.
There's nothing really fancy about it, we're using Volkuur's guidance to poison with fire damage and scale fire, as well as some crit multi for Perfect agony.
As for any league starter the goal is to get 4 voidstones asap while spending as little as possible.

It works ok and can kill maven fast enough, mapping is decent, but I was wondering if I could squeeze out anymore damage without overspending.
More importantly this character feels a bit too squishy, I do plan on getting some max res jewels but I'm pretty sure those would be quite expensive on league start. I'm not suppression capped, (50% + lucky), might get Magebane on the tree, but I think that's as good as it gets without more levels and/or paying a handsome price for spell supp on my boots.
Any advice there ?

Also mana is kind of a an issue, I might swap a support gem for inspiration, but I'd rather find something else. It works as it but I'd like to free up some mana related affixes on my rings.
Not ailment or stun immune either.

Finally I've been using a Toxic rain of withering setup to stack wither faster, but I've read somewhere wither stacks only apply to damage dealt by the unit that dealt the wither (aka player-dealt wither doesn't increased damage made by a proxy)
I would swear my totem damage increases after I start using Toxic rain, but maybe it's just the stacks from my totems ramping up, I'm not sure.
Can anyone confirm this ?

Thanks for reading, any advice and hindsight would be appreciated.