I’ve answered @Seraphim755 on Discord, but just to correct this for anyone else reading:
For each additional creature summoned beyond the first, you suffer an additional disadvantage 2 on your action roll to invoke.
This means that your invocation roll suffers an extra 2 disadvantage per additional creature, not that it adds to the regular multi-targeting disadvantage.
…you do not need to make an action roll if the disadvantage normally incurred from multi-targeting is completely negated by your Multi-Target Boon Specialist feat.
This specifically calls out the negation by Multi-target Boon Specialist because that’s what the feat counts. Boon Focus is not mentioned in the effect because it does not count towards this negation. Boon Focus is a prerequisite because it allows you to invoke without rolling, not because it negates disadvantage.
So for the original example:
- 4 advantage from Boon Focus 3
- 2 negated disadvantage from Multi-target Boon Specialist 2
Summoning 2 creatures would be at disadvantage 2, which is negated so no roll is needed.
Summoning 3 creatures would be at disadvantage 4, which is not negated so a roll is needed. This roll is at advantage 2 due to the advantage from Boon Focus.
Summoning 4 creatures would be at disadvantage 6, which is not negated so a roll is needed. The roll occurs with no disadvantage.
…and so on.
I hope this clears things up.