Wow. Entities. What a mess. I really struggled with this. There's an example of a database-based module not using them and one that does use them. I think it boiled down to me that what I was representing were reports, which seemed very similar to nodes (in hindsight, perhaps this should have been a content type all along), so I should use entities.
As I look through examples, trying to figure out how to get my menu routing to work with my database entries, it appears that I need to set them up as entities. I say that, looking at the xmlsitemap and dbtng_example and content_entity_example from the examples module. So, let me back up and learn about D8 Entities. It looks like entities were introduced in Drupal 7 and there is a guide for that, but it's based on D7, so the concepts stuff is ok, but the implementation guidance is outdated. There's a link at the top to take you to a D8-specific guide. The guide says it's just a holder, but I'm going to give it a go. If nothing else, perhaps this post will help others.
First, entity types. Either you're setting up a configuration entity or a content entity. I think content entity fits for me. Next, looking at requirements section of the content type doc, I need to setup src/Entity/GatedContentEntity.php file with docblock stuff that will give the loader the information it needs. Hey, checkout this helpful hint about double quotes in the annotations doc (post #1 reference)! That annotations doc doesn't provide what all you need, but I'll take those 3 lines to start off with. I think this doc may list all of the properties you can use with ContentEntityType, but not sure which ones I really need and how to use them. This is when the examples/content_entity_example/src/Entity/Contact.php file helped. I first just used the core form & access handlers for now.
Random Aside: Sublime Text 3
I use Sublime Text 3 as my IDE and there is a PHPDoc plugin that makes life editing these annotations easier. Package Manager makes it easy to install plugins: just follow the installation instructions (and restart ST), then follow the usage instructions to pull up the Install Package option and then type phpdoc and select it to install. Then restart ST again and you'll be much happier. I think the same applies for Sublime Text 2, too. This Drupal doc has some other helpful Sublime Text tips.
Back to it...
So I found this great doc on how to create your own content entity. And whoa ... this comment pointed me to the Drupal Console project, which can jumpstart this a LOT. So I tucked my handcrafted code away and ran this command:
The end result is about 10 files with lots of boilerplate fields that don't match up with what I'm using. Maybe I'm going about this all wrong. Maybe I don't need Entities? Time to take a break...
After the weekend
So I went back and added the preface above and I feel I need to stick the course with Entities. Perhaps this post will be totally rewritten after I totally understand them and and write a better guide that flows better. I need to just release this post because I'm about to flip back to some D7 work and will be taking another break. I'll pick this post thread back up again shortly.