Short version. Cost per counted result (what you paid, all-in), net results after reversals, and the value of a result to you (margin times orders over the year). ROI is the third divided by the first, on the second.
The three numbers that matter
Cost per counted result (what you paid, all-in), net results after reversals, and the value of a result to you (margin times orders over the year). ROI is the third divided by the first, on the second. Everything else — reach, impressions, engagement rate — is a diagnostic at best.
Per creator, per placement
Because each creator and each placement has its own link, the three numbers exist at that grain. A creator whose bio link converts and whose story link does not is a creator to brief differently, not to drop.
What to do with the ROI
Rebook the creators whose cost per result sits under your value, invite them to private campaigns with a better reward, and let the automatic reward stop spending on the rest. The history of every campaign stays in the account: next season's roster is a sort, not a search.