What Finland should do, is "invade" Ukraine. And they take over all the areas that russia control. Take them in the name of Finland. So at this point, they are Finland occupied, but still part of Ukraine.
Now if Russia comes in, they risk engaging in battle with a Finland occupied territory.....a NATO territory.
Crimea might be a bit trickier......because it's no longer considered part of Ukraine.
But then after the war ends, Finland occupies all of Ukraine. But not in a threatening way. Just to make sure russia doesn't come back.
Now if Ukraine on their own want to take back Crimea, they can......but I can't see that being a NATO thing.
And why would Finland do any of this? Well........it's russia. Fuck em.