...OK, the changes that got it to work in ROS Humble is already implemented w/ the latest turtlebot3 repos, but I don't see a ros-iron-turtlebot3-navigation2 package. Seems my choice of using the cutting edge of ROS for this mini-class is biting me in the butt.
To confirm I ported the code to my Humble VM - and it works out of the box.
...I'm just going to stick with Humble for now. I'll confirm by having vagrant build me a ROS Humble box for the code to spot check and confirm.