Accessibility (A11Y) is going to be for the 2020’s what Responsive was for the 2010’s. Lots of lawsuits happening here in the USA, mostly large-cap(deep pockets) companies being sued.
Do they have a current site? Have you done any A11Y testing on your own on it?
First test would be to try and navigate without a mouse, keyboard only.
Next thing I’d do, is try it out with Chrome Vox extension. It’s a “screen reader” extension. Then try out the site with it and no mouse.
I know you said that you’re not interested in being certified, not sure who does that, but the WCAG guidelines are a great reference. There’s a great extension for any chrome based browser called Accessibility Insights.
It’s a bit tricky to use as it launches a second browser window, but you should start using it on every web site you work on.
Here’s a post I did before on using it with links.
Some of the A11Y stuff will be outside your control as a RapidWeaver user. It will be on the developers to fix something’s. If you find something that’s not compliant, let them know.