The presence of a Skipman in "Wax Museum" is accounted for.
In Dr. Frakes's lab, one of the magnetically distorted images she captured from her portal experiments is clearly a highly blurred image of the Owl House, from Dana Terrace's show of the same name. There have been long - running fan theories and gags (and a virtual panel/script read-through) connecting the two shows/dimensions.
At the audition line you can see an old man who looks like Muten Roshi from Dragon Ball.
"Primthistle Manor" is a play on Downton Abbey. Michelle Dockery, who plays Lady Olivia, starred in Downton Abbey as Lady Mary Crawley.
The pictures on Mr. X's fridge are depictions of his voice actor, RuPaul, in his iconic drag persona from the reality show RuPaul's Drag Race.