I don’t think either of these things requires more than a vote of Congress and the President’s signature. DC might, because of its unique Constitutional status and the current partisan Court, but there’s no argument for PR. You would have to abolish the filibuster, but Congress can do that once per session by majority vote — it’s just a legislative rule and not a Constitutional mechanism.
I mean even if we accept the premise the problem is if you start to engage with this game then the next Congress can do it too.
Pretty soon you'll have "Middle Dakota". And on and on.
At a certain point the USA is going to have to address its structural issues - the founders foretold of this necessity. It's why the amendment process exists in the first place.
Getting rid of the filibuster is a good thing in general. The GOP-led states have already moved into mid-cycle gerrymanders and routine gerrymandering of the state legislatures to eliminate fair elections there: I'm sure eventually someone will have the idea of adding new GOP states. Adding two actual territories/districts that are full of actual unrepresented Americans happens to be a good idea on the merits, since those people are being screwed by the Federal government.
