I want to become more advanced in writing in an "obscure" language.

My family refuses to teach me how to speak it because they explicitly said I had no need to. To be fair, we do not live in an environment where speaking it constantly is possible (them being in the province and us in the city).
I mean, I'm not doing this out of necessity and instead I see it as a hobby, challenge, and aspiration of mine.
I used to want to speak to them one day in that language, but now, I also want to be able to write creatively in it and see how different I would express myself.

That aside, how would I approach this, especially with extremely limited/non-existent resources?
Should I be more technical with the grammar? Like, how to form words and sentences.
Or, should I just proceed to learn phrases and pick up meaning along the way?

Btw, the language is Bolinao, a Sambalic language in Pangasinan.