By following this guide, I think I've managed to solve the issue. I work with 10.11.6 El Capitan. Let me resume: first and most important, I've added the third string to the PATH variable by typing in a terminal
sudo nano /etc/paths
then added the string
/usr/local/texlive/2019/bin/x86_64-darwinlegacy as last line
press CTRL + X to close and press Y to save changes. If you close and re-open the terminal window and type
echo $PATH you should see the new path added to the environment variable.
Similarly for the other two. Always in the terminal type
sudo nano /etc/manpaths
/usr/local/texlive/2019/texmf-dist/doc/man string, exit and save.
For the last one I have some doubts on the effectiveness, since my file was initially void. Anyway, as usual, type in the terminal
sudo nano /etc/infopaths
Finally, check that Tex Live Utility is reading the correct path (2019). Open that app, go to the preferences and point to the path of the 2019 version
Now Tex Live Utility should update the packages.