Another copyright /ISBN question

I was listening to the Author Update replay from this past Friday and I have a question.

Back in 2023, I followed the bad “Facebook Author Group” advice and didn’t get either the copyright or ISBN for my debut novel.

Question: Should I now go back now and get the ISBN and the copyright for the standalone novel I released in June of 2023?

It only has about 100 lifetime sales so I’m not sure if it is worth the time and money at this point. It was not my first book (I did not break the Ninth Commandment of Book Marketing), and I’m deeply grateful for the people who found it and enjoyed it, but I understand why it’s struggled to find an audience. It's a portal fantasy, but the "normal" world, which is usually Earth, couldn't be Earth, for story reasons, but it's weirdly similar to Earth, especially in the way cyber security works. I think it’s probably a little confusing to readers.

Would it better just to spend my time and energy on writing my next book?