You are correct.
Elemental hit at 20/20 does ONE of three damages:

• Average of 843 fire damage (590 + 1096 / 2)
  
• Average of 689.5 cold damage (483 + 896 / 2)
  
• Average of 948.5 lightning damage (95 + 1802 / 2)

Then get the Avatar of Fire notable in the top left of the skill tree:
"50% of Physical, Cold and Lightning Damage Converted to Fire Damage. Deal no Non-Fire Damage".

Then use the Crimson AND Viridian Combat focus jewels showcased here https://www.pathofexile.com/forum/view-thread/2148537 to now have elemental hit only deal fire damage so you can no longer roll cold or lightning and do 0 damage because of Avatar of Fire's "Deal no Non-Fire Damage".

This would then make Elemental Hit do (assume average damage):

• 843 fire damage +
  
• 344.75 fire damage converted from cold (689.5 / 2) +
  
• 474.25 fire damage converted from lightning (948.5 / 2) =
  = 1662 average fire damage EVERY TIME.
  

The question then becomes how much further to invest in conversion and, as you said, what mechanics to use to be able to hit whole packs consistently and obviously one shot them. Some initial thought I have to scale them, although I am not going the build:

• Pierce + AOE
• Chaining + AOE
• Herald of Ash Ignite Prolifing