Thank you, @pfaffman!
After updating and upgrading ubuntu, rebooting the server, running the doctor, and repeated retries, I was planning on trying this next when I saw your reply confirming those suspicions.
Disabled the non-Discourse plugins, and the rebuild was successful. Re-issued SSL certs are now in use, and I can take some time to get it all configured the way we had it.
Appreciate the assistance, and hope to spend more time here and learn more when not under the gun.