believe me, in France you do!

you need to declare on every purshase with btc (not buying btc). Ie if you buy a croissant:

1) how much btc you paid

2) which btc you used to pay (you décide) to know what was its fiat price at this time

3) you do some math : tokens price € when bought minus token price € when you paid

4) you can now pay 30% on the "plus value" or if you loose money it's ok you don't have to pay anything 😅