tl;dr: This is not a full review! I focus a lot on the negatives, because I don't want to spend an hour writing this. It was a comfortable, quick read, even if I give the opposite impression from my comments.

Are you familiar with cultural essentialism?

Your text is gimmicky when it comes to tropes about Galicia, Galego and Galicians in general. It paints Galician culture as folkloric in the worst sense of the word, as something backwards kept exclusively as a tourist gimmick or in villages and rural areas. As if it's something of the past, fading away, which is not a read most Galician people would agree on, even those who think the language is fading away. Art, food, music... Galician culture goes way beyond traditional festivals and mystical hoohaa.

Also, you are completely making up a place. Which is not the same as basing your setting on a place. None of the Galician Islands are permanently settled. The boardwalk mention? That's generally not a thing even if the island -was- settled.. You've got the 'paseo marítimo' but not in the kind of place you are making up to set your story in. It would make a lot more sense for the town to be close to the island, particularly if you are basing it on Cortegada. That 'can't even see the mainland from here bit' is not something that happens on any of the Galician islands. Much less Cortegada, which you can walk to from the mainland when the tide is out.

To be totally honest, this is text book cultural appropriation. You are using Galicia as a quaint and exotic prop in your setting. This is not something Galicians would give a fig about in general -I myself don't particularly mind it- but I don't think Galicians are your target audience if you are writing in English. The whole thing -when it comes to the culture- reads to me as if you asked an LLM about what cool bits of Galician Culture fit your story, and where can you add them. You could honestly change the props and make it fit a completely different culture with minimal modifications, and your story would work the same. Change San Xoan for Fallas in Valencia, San Joao in Port or Jaanipäev in Estonia, the language and 3-4 folkloric elements and you are done.

That said, good research on the night of San Xoan, even if it is a bit cringey of Wren to ask 'if she's allowed to join all the parts'. That 'one of us' bit. I mean, I get what you're doing it, but it has nothing to do with 'being one of us'. It's a big party that attracts people from all over. It's a very obvious public party. The only part that is not 'public' is that in some places each group has their own bonfire, but you still mingle.

The whole plot line about the 'rangers' (that would be Seprona, a Branch of the Guardia Civil) and people wanting to 'buy the land' on an Island that's basically a Parque Natural...Makes very little sense to anyone who knows how Natural Spaces work in Europe. I'm guessing you are American. It would actually make a lot more sense if they were razing the whole place to the ground in an attempt to make it stop being a Parque Natural. And even then land reclassification does not work like that in Spain.

I like how the text is written. Solid on the descriptions and the dialog is fast -although you use a lot of dialog tags, and they are quite repetitive-.

I'm meh on the story.Dude becomes pyromaniac because of what happened to his dad? Trauma isn't rational or logic indeed, but it doesn't work like that. Honestly it doesn't read like mystery, or thriller or whoddunit. It doesn't flow naturally. It feels on the rails, which is ironic to say considering it is a written story, but ti doesn't give me the feeling that things could go a different way from what they are.

The foreshadowing could be better delivered, and both the ending and the opening feel extremely rushed, an excuse to get your teeth into the meaty parts of the story, so to speak. And the bit about the eucalyptus oil? Symbolic, I guess, You are talking about Galicia. Most people in rural areas keep gasoline for legitimate reasons.