Hi @edwardsmarkf, @schoen describes fullchain.pem in another post..
https://community.letsencrypt.org/t/solved-why-isnt-my-certificate-trusted/2479
If you take a look at it he stated:
I don’t believe that fullchain.pem references the ISRG root. I originally wrote the code that originally creates it and the definition of fullchain.pem was cert.pem and chain.pem in a single file.
Also if you want to make it "simple" you can comment out the ssl configuration in your httpd.conf and "include" a separate file with your custom ssl settings, etc. It might make it easier for one to maintain and keep track of.
As to the editing of the fullchain.pem file, I'll leave that to the experts. That's why we're here!
Hope this helps a bit...
Rip