Public Transport from Ponte De Lima to Santiago?

https://www.rome2rio.com/map/Ponte-de-Lima/Santiago-de-Compostela

Bus or Train to Valença then to SdC.

What Sabine writes. Take the bus from PdL to Valença walk over the international bridge-built George Eiffel from the Paris Eiffeltower - and you are in Spain.
From Valença you can take the train back to Porto. Ponte de Lima is the oldest town in Portugal and interesting enough to stay for a day or so.
Also Viana do Castelo is interesting for a day to visit and from there you can take the train back to Porto. There is a busconnection between PdL and VdC
From PdL is a bus to Porto via Viana do Castelo and the coastal motorway.
A tip. Once in Ponte de Lima take the bus to Braga. More interesting than the south of Galicia if you have two days only in Spain Except for places like Santiago or Pontevedra there is nothing interesting in that part of Galicia and both mentioned places are far away.
Braga is an old Roman town and has the world heritage sanctuary Bom Jesus do Monte . Very nice to visit. And from Braga its easy to get to Porto by train.
Your second day you could go to Guimarães , a medieval town where the royalty of Portugal started about 8 centuries ago. Beautiful town. And easy acces by train in one and half hour to Porto

Bom caminho

I agree with Albertinho , after Ponte da Lima a visit to Braga is a wonderful idea. I spent a weekend there after finishing in Finisterre., most enjoyable.