Easter weekend arrival in Porto, start walk in Baiona

You can use Oficina de Correos to transport any luggage in Galicia see https://www.elcaminoconcorreos.com/es/transporte-mochilas There's is a PO in Baiona. It's closed on a Sunday.

The buses will be running but the weekend service is different than during the week.

Bars, restaurants etc are all open..it's the shops/supermarkets/banks etc which are closed on Sunday or Bank Holiday.

I would stay and participate in the Easter activities. It is a great cultural experience and start walking Monday.

I'm not sure about Portugal, but Easter Monday is typically a holiday in Europe and has the same 'closings and schedules as Easter Sunday'. I found this out the hard way in France several years ago... I'm not sure what was/wasn't open on Saturday.

Portugal has only a few days when supermarkets are all closed.
Sunday its generally open for shopping, but not all shops and sometimes with shorter working hours. Easter Sunday is one of the real holidays, so even big supermakets are closed after 2pm.

Although Spain has some localities, that open their supermarkets every Sunday morning, that's not the rule. It's rather complicated to say what happens on easter. Thursday, Friday and Sunday should be closed for business, while easter monday is a regular working day in Galicia.
I doubt anyone nows all the local holidays in Spain and how they apply to shops.

I enjoyed staying in Pazo Pias (A Ramallosa). Its only 15€ per pilgrim and you can count with central heating. It's about 1 hour walking from Baiona and for sure the bus from Vigo has as stop in the town as well. The Albergue in O Freixo has no heating and I saw no supermarket. You can still go there and take the bus into Vigo to find shops and ho(s)tels.