Fullness does not mean "full inclusion" of all the Gentiles, which is a "futurist" interpretation.In Romans 11:25, "Fullness" means full inclusion, rather than the futurist doctrine that there's a gentile quota to meet. "full affect" works, but is vague.
"Fullness" is adequate in that it's a literal translation, leaving us with the context to know what it means.
We can take fullness of time to mean when the time was right.
Fullness is inadequate because it is too vague, as your interpretation demonstrates. Intended affect better presents the message.
But thank you for your thoughtful response, especially the "leaving us with the context to know what it means."