If the transaction is done quickly, there is no capital gains, therefore no tax implications. You're buying and selling at the same price. If you keep crypto inventory, that's where it's an issue. This is why @v4vapp works well with Lightning. I put in cash, Lightning instantly converts to sats, which v4vapp instantly converts to HIVE or HBD.

And there are services such as Revolut.com and Wise.com that allow you to have multiple currencies.