If there is award availability, you can use United (or other Star Alliance carriers) points on Aegean domestic flights.
Austrian, Niki, British Airways, and Vueling also have seasonal flights that you can book with miles.
Niki, from Vienna, is bookable using American miles and some Vueling flights are as well. They should show up when you search on aa.com.
You don't say when in Fall you're going, or where in the US youre flyinh from, but especially for September, the best routing is the one that has availability on the days you want to go. If you have the points now, book the transatlantic flights now. No one can predict what flight availability will be after you build up more points in a few months.