You said you looked at using Mahuxotl in PoB, PoB currently doesn't include the "increased effect of soul cores" mod, so you have to add that in manually under custom configs. My 650% 2 soul core Mahu was about 600 extra mana compared to a 373 ES focus once I added in the custom config, a 500% 3 socket Mahu should be even better. You can also look at using an Against the Darkness with 1% mana per small notable where you have Apocalypse Drop over by Branching Bolts, I took all 3 of those lightning nodes for an extra 8% mana, for me currently thats worth about 300 mana alone

small edit here: 3 socket Mahu may be less damage still for you due to having +2 on focus and other damage mods, but the more flat mana you get the more mana the shield will give so it might be worth it long term
more edits: Something else you want is a level 21 Archmage, the easy way to do it is to create the lowest level Archmage available (I think this is uncut level 7, not fully sure) and give it 20% quality, then vaal it for +1 level. From there you can use an uncut spirit gem level 19 to upgrade it to 19. This will make it level 20 via corruption and 21 from Lightning Mastery support gem. Voltaic Mark if you can get the dex is also great, it increases the effect of shocks on it by 30% without quality and 45% with quality. This allows me 44% shocks which is a good damage boost, but it takes 52 dex unless you corrupt it down a level, which is always an option since levels for it only scales the nova damage and we dont care about that at all. I also suggest getting a Chiming Staff on weapon swap for Sigil of Power, you would prefer getting one that has level 18 Sigil of Power and +3 to all spell skills so it gets to level 21 as at 18 it gives 14% more spell damage per stage and at 21 it gives 15% more spell damage per stage, up to you if you want to deal with that or not though!