I have CZK as my base currency. But for tax purposes (AFAIK) you usually get the data (crypto prices) in USD and then convert to CZK using one of the allowed official conversion rates. There is an official (set by the Czech state) yearly or daily rate of USD/CZK.

Koinly dosc https://support.koinly.io/en/articles/9489964-how-koinly-sets-the-market-price-for-your-transactions say:

“If your base currency is different from USD then Koinly will apply additional rate conversions to determine the market value of USD in your base currency on the date of the transaction.

The price source for the conversion of USD to Base currency (ex. EUR, AUD, GBP etc) comes from sources such as the European Central Bank. The rates are synced once a day.”

But it seems to me that it is not helpful for taxes in Czechia. Ideally the user would choose the conversion metod - either daily or yearly official Czech tax office rate. Choosing it allows for some perfectly legal tax optimization.

Currently I think the correct option for me is to use USD as my base currency and do the conversion manually. (Which is easy for the yearly rate, but difficult for the daily rate)

An article (in Czech) explaining jt more: https://www.behounek.eu/l/jednotny-kurs/