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 😅