To improve, I'd like to read game records of professional players, and to understand these commentaries would be useful. Do you know if these game commentaries exist, and where I can find them?
I kept searching for the past few days, but the only thing I found was tenhou's logging feature. It basically lets you replay previous hands, so you "only" need to find strong players and links to their hand records. I do not yet know how to do that.
Example: http://tenhou.net/0/?log=2008120919gm-0007-7447-xc9f52854acf6&tw=3. It starts with some Japanese advertisment, but just click the link at the bottom after a few seconds and you'll be lead straight to the game. Everything is self-explanatory from that point on. Sadly, I have no idea how strong the players in the link are, I just found it by accident.
Apart from that there is a number of youtube videos, but it's rather hard to follow so I can't recommend it really.
Sadly neither of these options offers commentaries, so it's only partly useful.
Replays on tenhou look like this: