tl;dr: The best strategy to enchant armor with an auto enchanter is to first enchant it level 30 and then a ton of automated level 1 enchants.
I was messing around with this in creative and I found that enchanting something multiple times takes exponentially more work for each enchantment.
Then I found that this doesn't apply for level 1, this always takes the same tiny amount of work, no matter how many enchants.
So I built a loop with item ducts that would enchant my chest plates over and over (I used amordine here). After about 2 buckets of mob essence they wouldn't get any new enchants but at that point they'd have Protection, Absorbtion, Unbreaking, Fire- and Blastprotection 3, Thorns 1, Projectile protection 4 (always exactly the same).
From this I conclude that the best strategy is to get a good level 30 enchant first while the price is still reasonable and hope for a good high level enchantment like Absorbtion 5. Then just keep going with level 1s until you can't get any more out of it.