Reddit reviews Colloquial Cambodian: The Complete Course for Beginners (Colloquial Series (Book Only))

Here are links for the Thai, I used this one for Khmer (not the best). I also used textbooks from the University of Washington and SEASSI. They might tell you where you can buy the resources even if not in the program. (SEASSI is a great place for heritage speakers to do intensive summer language programs, btw, and they offer scholarships - or did - when I went years ago.)

I think all of the languages work really well with abugidas, so I would stick to that. Lao is probably the best script for an interlingua if the goal is simplicity and basic communication. But any of the three scripts could be pared down to basics - and for that, Thai is the most widely understood by all three language speakers due to its economic and cultural dominance. Just my two cents!