I think the OP did well. He had an exit point that guaranteed considerable percentage profit. In retrospect it is easy to look back at price movements and say that you could have sold at a higher point, in practice it is harder, and it's next to impossible to sell at the absolute peak. When prices fall after a sustained rise, the fall is often dramatic. It is in general much better to sell in a market that is still rising, as the OP did.

And of course given the dollar price volatility of bitcoin, whilst he sold at 30k and the price is now nearer 50k, can any of us really say with confidence that the price will never drop back below 30k again? If/when this happens it would obviously give the OP a lower entry point and the option to increase his bitcoin holdings compared to his original sale point.

If it does happen, he can increase his quantity of bitcoin. If it doesn't happen, he's already doubled his money and is happy with the outcome.