At which stage of enlightenment are the aggregates seen as not-self fully?
The conceit ‘I am’ is the 8th fetter; only fully eradicated by an Arahant.

Bhikkhus, there are these five higher fetters. What five? Lust for form, lust for the formless, conceit, restlessness, ignorance. SN 45.180

The stream-enterer has overcome 'identity view' ('sakkaya-ditthi'); which means seeing the group (body; kaya) of aggregates are not a self. However, the stream-enterer's seeing of the aggregates as not-self is obviously not complete enough to remove all ten fetters and uproot all outflows of defilements &amp; conceit.
Therefore, arahant is the stage of enlightenment where the aggregates are seen as not-self fully.