So everyone there speaks Spanish, in Santiago and around people speak more Galician at home but everyone is bilingual. The accent is a bit closer than in Madrid, a bit like Portuguese from Portugal, if you are familiar with it. It has a melody to it, it should be easier than the accent from Andalusia. I suppose you won’t have issues to communicate and people will definitely appreciate that you at least try to speak Spanish. I would say that even with a bad Spanish level we kind of understand a lot.
about seafood, if you like it, give a try to “percebes” they are suppose to be a delicatessen, other typical dishes: lacón con grelos, carne ao caldeiro, caldo gallego, padrón peppers (Padrón is a village there). Alcohol: crema de orujo, licor café. Desserts: milhojas, tarta de santiago.
also Ourense has natural hot baths in the street in the city. I’ve never used them but it is definitely in the bucket list. During all summer every village has a gastronomy party, check the ones available in the dates you are going, every day a different dish in a different village, gastronomy is a BIG part of Galicia, and they have their own typical local dishes :)
I am from the south so I am a bit more familiar with that area, but I am sure you will like anywhere there. If there is another heatwave it will definitely be hot anywhere in Galicia.
I hope you have fun!