I was wondering whether the HSK collections had all the words, guess this answers the question. Personally I’d expect a collection of sentences that’s supposed to match HSK levels to have most if not all the words (though I understand there may be limitations with using Tatoeba as the dataset), so “completing” a collection would be more representative of your actual learning progress in relation to the HSK.
As a stopgap measure, I wouldn’t mind if the sentence restrictions were loosened if it gave me a more complete exposure to HSK vocab. It may be not ideal, but difficult words can be ignored or inferred from context, especially if they’re nouns, and we have the translations to compare them to anyway.
(On a side note, I’ve also seen quite a few sentence in the Fast Track collections with more advanced vocabulary than expected (one of them included something like “palm tree” for example, while the clozed word was much more common), I assume this is also due to the difficulty of splitting words.)