Do you need to change the directory, or you can just leave the cron job to execute the PHP script witin your folder?
So this instead:
/home/"my cpanel username"/public_html/core/php artisan schedule:run >> /dev/null 2>&1
Also you can add Log::info('Testing'); in your Kernel.php file above the $schedule->call calls, and make sure that the line is getting printed in the log, that way you'll know if your scheduler is working. Nothing else seems wrong from a first look.