Wow, for a week that is an aggressive itinerary.

I'd decline to discuss Thailand as it has been 5-6 years since I was there and 10+ years to south VietNam (Hanoi & Halong Bay in September 2018).

As for Cambodia, I'd skip Phenom Penh and go right to Siem Reap (from Saigon if possible) and just plan a day or two doing the Angkor Wat museum and temples (Angkor Thom etc).

Flights are cheap between those 3 countries so I'd pass on busses based on your limited time. I think my flight from Hanoi to Siem Reap (vietnam airlines) was US$80 and from Siem Reap to Singapore on Scoot about the same)

In Siem Reap, I stayed in the heart of the CBD at the Cheathata Suites Hotel with rooftop bar/restaurant, private driver,and safe/clean for about US$50 a night.

I hope that helps a little