Anything that requires a constitutional amendment is a total non-starter. Instead we should just immediately grant statehood (like we've done 37 times since the original 13) to DC and Puerto Rico, since they've already voted on it, conditionally pre-grant it to the Marianas so that as soon as they vote on it they can become a state automatically, and open a dialogue with USVI and American Samoa to see what they want in the medium and long term. Perhaps something can be worked out.
The DC Admissions Act as it stands requires the abolition of the 23rd Amendment, so that the remaining residents of the rump capital territory don't retain three electoral votes. By all accounts, if it passes, the amendment should sail through state legislatures with little attention, since it's in both parties' interests.
So, if and when PR is granted statehood, I can totally see a Republican Party that, in order to solidify the Puerto Rican vote both in PR and the mainland, champions an amendment to guarantee the status of the Spanish language on the island, and that the amendment wouldn't face any opposition from Democrats.
You're right, that a bespoke arrangement for the four remaining territories would be very difficult to implement, given the existing political realities.
