Hi there.....
We live in an apartment as well, been here, happily for just over 3 years.
Mishka my AG is 2 1/2 years old.
Mishka has loads of toys, to keep her occupied and stimulated. She has won the hearts of the tenant, who live close by. When on the balcony, she has a large audience (which she loves) LoL
When I am outside, hanging washing etc, I can hear Mishka talking and playing. She has her moments of course. I reckon no different than a baby, on a bad day.
Mishka in general in not a loud bird, loves talking and whistling. Her whistling can become a bit loud, but thank goodness, she whistles beautifully and in tune.
Not familiar with the sun conures, how loud they are etc
Sun Conures guardians will be able to assist you, pertaining to their loudness etc.
Let us know what you decide.
Good luck