Last Fr Davis R Users’ Group, Jaime Ashander gave a presentation on and visualizing and diagnosing (G)LMMs in R. Here’s the video:
Jaime also wrote up the notes from his talk, including all the code, on his blog here (with the raw R Markdown file on github here). The material in the blog post is expanded and improved upon from the original talk, though this means the video and posted code don’t match exactly.