I have done something similar; set all the phones with no 'Fwd No Answer'; let the lines be answered with Auto Attendant like you said, at 8 rings; then use the 'Transfer Callback' system programming parameter, set to about 4 rings.
This way, when the lines ring A&R to all sets, Auto Attendant answers on the 8th ring, then when the caller chooses a local extension, the Transfer Callback timer starts (1 ring, 2 ring, etc) up to 4 rings, then the Transfer Callback feature kicks in, having the local extension number that the caller entered, and 'rings back' to Auto Attendant with the local extension number, and drops into that local extension's mailbox.
Hope that's what you're looking for.