One measure would be feedback forms gathered during the engagement themselves assessing participants’ satisfaction.
I would also check in with community influencers. From my own work, I have found that there are always local community spots where people share their thoughts informally. Currently, medical centres where Indigenous residents go to access virtual care or vaccines are a place where people share their thoughts. Ask people in those settings how residents are feeling.
I think the main principle is to approach engagement with Indigenous communities as a long-term relationship and the right “measure of success” will emerge.