A wild suggestion: Try voice-acting a variety of characters from TV or books.
As a non-Japanese-native, one thing which helps me shed my “foreigner’s accent” is by rehearsing ‘Kamehameha’ and similar punch lines from Dragon Ball as a kid. Pick some of your best characters in one of their best moments and try your best to sound like them. It helps over time.
If you’re willing to invest the equivalent of a few hundred Euro of your time, the Ward Audiobook Project and many others are always looking for volunteer narrators.
LessWrong also has a textbook repository which contain recommendation on various subjects. Ctrl+F “decision theory” turns up 6-7 books or so.