Got it to work 🙂
I turned on wp debug as you directed and from that it indicated that I needed to include the following file in my child themes includes folder as well: class-tgm-plugin-activation.php
So for anyone else looking to do the same.
#1. Edit Cardinals functions.php file as follows on line 40:
/* include_once(SF_INCLUDES_PATH . '/plugin-includes.php'); */
include_once(get_stylesheet_directory() . '/includes/plugin-includes.php');
#2. Create an includes folder within your child theme and copy both these files into it:
plugin-includes.php
class-tgm-plugin-activation.php
#3. Edit those files as desired.
So, considering what I have set up now, can you confirm that the only file I will need to continuously manually edit on theme updates is the functions.php file as out I laid out above? (there is definitely no way around this?)
Thanks.