If you have reached your bitcoin target does not mean that it's selling time, rather you have just transit to your maintenance stage. You can only think of selling part of your bitcoin when you have accumulated more than enough Bitcoin.
After achieving your goal, an investor can certainly sell a portion to enjoy the gains, or plan to reinvest elsewhere, which I think should be done. Because it would not be wise to put all your eggs in one basket. However, that doesn't mean that after reaching your goal, you should sell all your Bitcoin—no Bitcoin holder would want that, especially not me. This is because you would never want to sell a company that you’ve built with your hard-earned money to someone else. Investment is like owning a company or a business. Once established, it will continue to provide you with profits for a lifetime. You could even pass it on to your heirs if they are capable. Just like you would never be willing to sell a company you built with effort, holding onto Bitcoin is like holding a company, and completely selling it might be a wrong decision.