There are plenty of options. The Red Sox could also resign Kelly and give him the role. The Red Sox don't need a long term closer--they already have that locked up in Feltman. The Red Sox need a bridge closer, a guy who can do the job for one season until Feltman is ready in 2020.
I would sign Britton to a one year pillow contract. Throw extra cash in the contract to entice Britton to sign a one year deal. If he rediscovers his dominant form, the Red Sox can get a pick for Britton next year.
The thing that many people are overlooking is just how well Dombrowski has rebuilt the farm system. The Red Sox must have the best farm system in baseball that no one talks about. The Red Sox have several future major league starters in the system, Groome, Houck, Shawaryn. A future closer in Feltman. And future infield starters such as Casas (long ways away), Dalbec (could be ready in 2020 for 1b or 3b), Flores (long ways away), Chatham (perhaps a future starter at 2b, could be ready in 2020), Ockimey (could be ready in 2020), and Lakins (profiles as an above average set up reliever).
It wouldn't make sense for the Red Sox to offer Pearce anything more than a one year contract; Dalbec and Ockimey will be cheap options at first base in 2020. Moreland has only one year remaining on his contract.
While the Red Sox have some promising minor league infielders, their minor league outfielders really suck, it is a weakness in the organization, but two of the three OF spots are locked up long term: Benintenti & Betts. I'm not sure about Bradley. For some players, defense declines rather early and Bradley's bat doesn't carry him.