Elemental Hit Ballista Champion with Wilma's was my favourite build of Sanctum (SSF btw) and it gets massive buffs for 3.21:

• ~~Reliable "covered in ash" via Xoph's Blood buffs~~
• +2 Proj from tree
• 100% increased Damage with Hits against Ignited Enemies
• Transfer Mark Mastery
• +1 max all res (from Reservation Mastery)

The character had around 8m shaper dps and manages to hit 13.5m shaper dps with changes (if 3.21 dev pob is correct).

~~3.21 pob:~~ ~~https://pobb.in/sv4xzny_QshC~~

3.20 pob https://pobb.in/Xp2AEaMGUvce

3.20 character: https://poe.ninja/challengessf/builds/char/Totämöggerli/Angustin_tFVGfopOQq?i

Edit: Xoph's Blood probably does not get changed at all. The patch notes only refer to Xoph's Heart.

Updated 3.21 pob: https://pobb.in/zmy9bHjwXrI2

Use Skitterbots before Wilma's or if you need more unreserved Mana. Vortex can be added once you drop Skitterbots for Chill (10% more damage). A veiled "Shock nearby enemies while focused" could be added to enable shock.