I’ll give it a try.
This line installs uhubctl, it’s only done once, the sudo (Super User Do) just allows the apt-get command to be done.
This line puts you in the directory zynthian/config - only done once, but Zynthian knows to look there duriing startup.
Remove (delete) the file zynthian_custom_config.sh so we can create it cleanly.
Create the file zynthian_custom_config.sh with the single line which invokes the nuhubctl command. This is the whole point - this command will be done at every startup, because the file we created is run at every startup.
Make the file we created executable. Just done once.
I’m not all that certain this will do what you want, but I thought it would point you in the right direction - you can put whatever commands you find work in zynthian_custom_config.sh and Zynthian will run them at every startup.