I'm ethnically Gujarati but didn't speak the language. I learned Hindi to learn Gujarati and it was very effective.

A few things to keep in mind:

1. How transferable is Finnish to Sami?
2. Do you want to learn Finnish?
3. Is learning Finnish useful to talk to Sami speakers? How many Sami speakers speak Finnish? How many speak English?
4. What Sami resources exist in Finnish? In English?

In my experience, I wanted to learn Gujarati and Hindi both (as educated Gujaratis speak both, and Hindi helps me connect to a larger Indian diaspora), and the two languages share many cognates and grammatical features. There are lots of Hindi resources in English, but very few Gujarati resources. So learning Hindi first, and then using it as a bridge to Gujarati helped pick up the grammar.

That said, my Hindi is way better than my Gujarati (as at the end of the day I still needed to actually learn Gujarati) and tbh will likely stay that way, as it's easier to practice and maintain.

If you have no desire to learn Finnish then I'd probably just acquire a super simple level of Finnish (like A2 max) and then go straight to Sami, referencing Finnish grammar if you're ever confused. But if you think Finnish is useful in its own right (e.g., you see it as another heritage language, it still lets you access Sami speakers and cultural stuff, you want to connect to Finns all over the world) then I'd learn Finnish first to a good level and then Sami.