Monbus only has one bus a day from Santiago to Sarria I believe. Otherwise you bus to Lugo and then catch a bus from there to Sarria. There will be more daily options for this. The current schedule and the July schedule will be similar.
https://www.monbus.es/en
I often suggest walking the last part of the Camino Primitivo from Lugo for folks wanting to do 100 km to Santiago. The Primitivo joins the CF at Melida but there are alternative ways to Santiago. I say suggest instead of recommend because I've not done this walk myself. Lugo has a two kilometer walkable on top Roman wall encircling the old city.
Details at this forum thread:
https://www.caminodesantiago.me/community/threads/lugo-to-santiago.77492/
After Santiago you may enjoy walking to the ocean at Finisterre, another 90 km, usually done in three days and more often in four than two. Decent bus service back to Santiago.