Hi Pauluc,
Some 15.1 versions are or will be EOL soon as you mentioned. However, for EX2300 and EX3400, there is an exception. This is included at the end of the document:
""15.1X53-D59x (including D59, D590, D591, …): EOE extended to 27-Mar-2020 and EOS extended to 27-Sept-2020 for EX2300, EX3400""
https://support.juniper.net/support/eol/software/junos/#6
As you are now running 15.1X53-D56 you can pick 15.1X53-D591
Hope that helps!