But the more I look into it, the more I wonder: if one Bitcoin can be split into 0.1, 0.01, 0.001 or even smaller fractions for transactions, doesn't that kind of undermine the whole “fixed total supply” idea?

Also, sure, cryptocurrencies are decentralized in theory and the transactions are hard to trace. But at the end of the day, anyone holding crypt is still very much under the control of governments. You can't magically escape regulations or enforcement just because the currency is "decentralized." It's an interesting tension between theory and reality that I don't think enough people talk about.