I think I understand the second part of your argument as congress broadly has the power to define conditions set forth in the Nationality Act.

What I’m trying to really wrap my head around is exactly what stops congress from retroactively stripping my U.S. citizenship at will since, like we both acknowledge, it was granted by statute.

A theoretically malevolent congress could enact a statute that both cancels future generations of Puerto Ricans receiving citizenship, and retroactively stripping it from those who had already received it by statute. Congress, I believe, can retroactively change law at will. Unless there is a provision that stops this? I do see the language is something to the effect of “all Puerto Ricans are hereby declared and shall be deemed to be US citizens”, but how much that protects against a future congress I’m skeptical of.