“…when that fuzzy diving line is crossed”

I think there are circumstances when the line is clear. If your child is born in the US, you’re an American. If you’ve held down a job for a decade, you’re an American. If you belong to and donate to a church that actively supports its broader community, you’re an American. If you love the Never Ending Pasta Bowl at Olive Garden, you’re an American.

Not legally, but spiritually.

Right. If you don't want reactionary politics, don't put reactionaries in charge.

I think the problem is that amending the constitution is too hard.

Many states have an amendment process involving ballot measures, where a supermajority of voters can amend the constitution. This is not without its problems, but amendments pass more often, and it has the attractive aspect of directly consulting the citizenry.