A good strategy is buying GBTC whenever premium is less than 15% and selling whenever it goes over 85% premium. Rinse and repeat.
That sounds wise. Premium ended today at 43%, so we still have some more premium compression to go before I'd consider buying/covering. I do think that the range going forward might be lower than the 15-85% you suggest, because very few U.S. investors had previously known that they could trade the zero-premium European alternatives, and more and more will know over time.