Please follow the instructions in this TurboTax help topic to report your cryptocurrency capital gains and losses.

Hi, if I only make a gain loss trading cryptocurrencies, is it mandatory for me to enter it in my taxes next year? (USA)

Eg.
- Bought 1 BTC 2 years ago for let's say $ 5,000
- This year I sell 1 BTC for $ 8000 and right after that the price goes up

- I use all the $ 8000 to buy BTC back and get less of it than my initial holding (eg. 0.8 BTC)

(I know that it can be carried over into the following year at my advantage but I would prefer not to as I don't plan on cashing out any other investments any time soon)

Thanks!

There is no carrying over.

Once you sell you owe tax on that capital gain of $3,000. What you do next has no bearing.

It is short term or long term depending on your holding period

2 years is a Long Term holding period.