Regardless of perspective on whether a player should be able to restrict to base or otherwise, there is a clear disadvantage depending on familiarity of cards in that a player with no expansions should theoretically always do better than an equally skilled player who has expansions and isn't familiar with the cards that expansion contains. This advantage no longer exists once the player with expansions is familiar with their cards and their potential interactions. Essentially, I paid to put myself at a relativistic disadvantage. (oops)
Since, as you've noted, my only recourse would otherwise be to create a new account or wait 31 days (which are completely accessible solutions that make no difference to anyone else, just are very inconvenient for me), I don't see a reason why I'd not be able to reverse this.
If you'd be willing, I would like to pursue a solution to this, whether it be undoing my subscription (I'm fine with Shuffle keeping the payment, as I'd like to support dominion.games anyway), "preserving" it as you suggested, or transferring it to another account (I could make a new account to apply it to-- this would essentially be the opposite of the solution you suggested wherein I would create a new account to play base on. In this case, I would have no subscription on my current account and would have a subscription on said new account).