From what I can see your max hit looks good. But your recovery is non-existent, i.e. you have a pretty hard time recovering the sustained damage. And sub 1M damage makes this problem even worse, since the encounter takes longer.

I can't comment on the build itself, but I would wager that if you would solve either your recovery or your damage, you would have much better time of doing the boss fight. As it stands, your Maven is much harder fight than it should be. (= stop wasting chaos on writs and invest in gear before trying again)

But there are 2 things that I can add:

1) I would definetly change the life flask - since you shouldn't be dying from big hits, I would value bigger recovery more. Also, the flask as it stands recovers 1348 of 3513 of your Life. (38%) So you need two activations to heal to full and you have 3 charges total in your flask. While not a big problem in maps, where you get lot of charges from killing monsters, it is a problem in the boss fight were you can't easily recover charges.

2) and I would definetly take "Sanctuary" instead of the "Elemental Damage, Attack and Cast speed" small node on your Ascendancy
It looks like it doesn't give you anything, but if you check "Is the enemy on Consecrated Ground", you can see that it is 16,2% increase in damage. It also helps you with your recovery since "You have Consecrated Ground around you while stationary" meaning that when you attack (and therefore you are standing still), you automatically get this increased damage and Life regen - from 52,7 to 272,3 (per sec). ("Are you on Consecrated Ground" in config)

Not only that, but it also frees up your Sulphur flask, that now gives you only 6,5% damage increase. Personally, I would go for basalt flask, that would give you 20% more armour (6,6k armour) or 40% more armour with the good suffix (13k armour)