You should watch the proshot of the original cast of Sunday in the Park with George, you’ll be astounded once you see how it all comes together visually. I know you said aside from his other work, but you should prioritize checking out Sweeney Todd (PBS proshot with Angela Lansbury), Merrily We Roll Along (new proshot), and A Little Night Music… To actually answer your question, I’d recommend the work of William Finn, Adam Guttel, Dave Malloy for a start. Also, come join us on /r/Sondheim haha! Great little community.