You are not in a hurry, are you? If you aren't in a hurry, take it easy!
The 17:00 bus from downtown Santiago de Compostela to Lugo is scheduled to arrive to Lugo at 18:35. To plan to take the 18:35 bus from Lugo to Sarria means you hope either the bus from Santiago de Compostela arrives ahead of time or the one from Lugo to Sarria departs later...
As I see it, even if you took the 17:00 bus from downtown Santiago de Compostela, you would likely end having to take the 20:45 bus from Lugo to Sarria so there's no need to stress yourself trying to take the 17:00 bus (what would mean taking the 16:30 bus from the airport to the bus station on the downtown -scheduled travel time 20 minutes- or a taxi a few minutes later... decide yourself if that would be feasible...). Take it easy and go for the 18:30 bus to Lugo that, in fact, you could take at 18:40 at the airport itself (unless it got full at the downtown, i.e.) avoiding having to go to the downtown. It all goes as scheduled you would have 15 minutes to change buses in Lugo what should be enough time (see below for alternatives in case it wasn't enough time).
The 17:00 bus from downtown Santiago de Compostela to Lugo doesn't stop at the airport of Santiago de Compostela, just in case you were wondering.
If you miss the 20:45 bus, you could take the 21:35 train.
There's a Regional train from Santiago de Compostela to Ourense departing at 17:25 that would allow you to connect with a later MD train from Ourense to Monforte de Lemos (take a look at the web of RENFE making a separate search for each leg). Once in Monforte de Lemos, you could take the last bus to Sarria. Take a look at the web of Monbus for info about schedules. But this way you won't arrive earlier than going changing buses in Lugo...
If you are a group or if you can make a small with other people, you might ask at
www.busperegrino.com what they can offer you (fares and schedules).