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
add the /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
and add /usr/local/texlive/2019/texmf-dist/doc/info
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 /usr/local/texlive/2019/bin/x86_64-darwinlegacy

Now Tex Live Utility should update the packages.