@sindrome73 You say you don't think the problem is with MYSbootloader, and you think the problem is with the .HEX files that write the new Bootloader.
Those .HEX files are MYSbootloader. Channel 76 is hard-coded in them.
You can’t get round this by changing anything in your controller.
If you want to create and load MYSbootloader .HEX files for a different channel then you need to modify the configuration files you mention, then compile new .HEX files.
That is not a simple task, it involves multiple steps including the installation of various tools.
My advice would be to get FOTA working using the default channel 76 before trying anything else.