The following is REALLY pushing it so you might want to condense on this itinerary:
If you start at Onset, head to P-Town (1 day)
I usually go from P-Town to Gloucester (and then Isle of Shoals and usually Peaks Island) but I have more time than you. So, I'd say
go from P-Town to MDI (SW Harbor or Somesville). Problem is that you might have to wait for a good forecast for that. (Days 2-3)
Relax and recover on MDI and take free bus (remainder of Day 3 and Days 4-5)
Head to the Eggomogin Reach. Stop brefly at the Wooden Boat School (and possibly Bucks Harbor where you can use the excuse of getting some
fuel to go ashore and stretch your legs). Pick up a YC mooring at Castine, look around, and then take the boat to Smith Cove. (Day 6)
Go to Belfast. Spend night there or head possibly to Warren Island (if you do the latter arrive early enough to pick up a free mooring) (Day 7)
Head towards Fox Islands Thorofare (possibly with a stop for a couple of hours at Camden) and go to Perry Creek or Carver Cove or (several
other places like - if you are adventurous - Seal Harbor) ). En route you could stop at North Haven for a couple of hours. (Day 8)
Head towards Rockland and have lobster at the Rockland Cafe (Day 9)
On to Tenants Harbor (Day 10)
Back to P-Town (Days 11-12).
Then Onset (Day 13 out of a 12 day window!) I suppose you could go straight from Tenants to Onset but that would REALLY be pushing it -
and you'd need to have the current with you in the CCC.

Innumerable variations on the above are possible. Frankly, I wouldn't want to do his - I like to linger.

Jerry