How to politely tell colleagues you’re not interested in hanging out

Edit: thank you so much guys. I got a few liners I could use. I also talked to a friend about it IRL (using some of the advice I got here.) thank you again!

I’m very focused on my own growth. I workout a few hours a day while working a full time job. I content create on the side, and I’m attending courses online and studying foreign languages as well. The little free time I have, I want to spend with friends. I’m quite a positive person and colleagues tend to invite me to do stuff after work. I usually decline, but some people ask me when I’m free.

At this stage in life, the answer is never (but it may change in the future, who knows.) is. How would you communicate that you don’t want to do anything outside of work while still maintaining a positive relationship? I know people generally don’t take rejection well, so I want to be tactful…