[RPG] How to optimize Heart of the Storm bonus damage

Heart of the Storm, from an Unearthed Arcana:

Whenever you cast a spell other than a cantrip that deals lightning or thunder damage, a stormy aura surrounds you. In addition to the spell’s effects, creatures of your choice within 10 feet of you take lightning or thunder damage (choose each time this ability activates) equal to half your sorcerer level.

The concept behind the build is to have a sorcerer that's dangerous te be around: handling so much arcane energy, enemies get hurt merely by being close to him.

How would I get the most out of this bonus damage, with the following constraints:

1. The build has to include a way to boost AC/HP, in order to stay in the front lines (against 'standard' enemies/medium difficulty encounters, definitely no need to melee a Tarrasque).
2. If multiclassing, no more than 2 levels can be taken outside of Sorcerer.
3. Must take the Warcaster and Sentinel feats (unless there's a more reliable way of boosting AC, casting in melee, passing concentration checks, and casting spells as Opportunity Attacks). Sentinel + War Caster seems to be the only way to cast more than one ≥L1 spell per round (to trigger Heart of the Storm).
4. 20 Cha by L20
5. Must be Elf or Half-elf (unless this is what prevents an optimized build).
6. Encumbrance rules apply, so, Heavy armour might not be possible.

Here's what I have so far (feel free to deviate if you see any clearly non-optimal choices):

Stats (after Half-elf racial modifiers)

• Str 12
• Dex 14
• Con 12
• Int 10
• Wis 14 (to MC Cleric, for Medium armour/shield prof)
• Cha 16 –> 20

Level progression

• L1 Sorc
• L2 Cleric (Knowledge, for flavour)
• L3 Warlock (Fiend, for Command, Dark One's Blessing, Eldritch Blast, and Hex 1/short rest)
• (Remaining levels are sorcerer, choices added for completeness' sake)
• L6 War Caster
• L12 Heightened Spell (Metamagic)
• L10 Sentinel
• L14 +2 Cha
• L18 +2 Cha
• L19 Extend Spell (Metamagic)

Spell selection

1. Cantrips: Sorcerer (Lightning Lure, Shocking Grasp, Fire Bolt, Ray of Frost, Acid Splash, Mage Hand); Cleric (Spare the dying, Sacred Flame, Guidance); Warlock (Eldritch Blast, Chill Touch)
2. L1: Warlock (Command, Hex); Cleric (Protection from Evil and Good, Bless, Shield of Faith)
3. L2: Blindness/Deafness
4. L3: Lightning Bolt (couldn't find a lower-level Lightning/Thunder spell worthwhile, hoping to be proven wrong in this respect. I worry about Thunder spells attracting attention from enemies)
5. L4: Banishment, Dimension Door, Greater Invisibility, Storm Sphere* (comment further below)
6. L5: Dominate Person, Hold Monster
7. L6: Disintegrate, Investiture of Flame (Fire immunity/cold resistance; auto 1d10 if creatures end turn adjacent to you, helping trigger Sentinel and reinforcing the build concept, and the action replaces cantrips for the duration as more reliable damage, given multi-target and half damage on a save), Sunbeam.
8. L7: Reverse Gravity
9. L9: Meteor Swarm, Wish (mostly flavour choices, since you only get one per day, and can't cast at higher level slots).

*Storm Sphere: While an AoE/control spell that fits the theme, I'm not sure if it's an optimal choice—the damage is unimpressive, and I've yet to find a way to use it, trigger Heart of the Storm, AND keep enemies in the area for more than 1 turn (without casting it on top of myself and getting caught in the initial Str saving throw).

Any insights welcome!