What additional documents do I need for my Muay Thai training visa application in Cambodia?

Why would you need business documents and proof of income? Isn't that what the soft power is for? You're just going to confuse them

You're either applying for the visa through the soft power (Muay Thai in your case) or the digital nomad option. I don't think you can choose both at the same time and it will most likely just confuse the staff. Also you can't get One Visa on two passports? So why are you bringing both, you need to decide which passport to apply on.

You don't need health insurance.

I don't know about the Cambodian embassies but generally residence documents in Thailand are not required either.

A copy of your passport, Cambodian entry visa, local accommodation and flight details to Thailand.

You need photos of yourself

Possibly a short intro letter stating you are applying for the soft power Muai Thai training category. Don't know why you're including business related documents on a soft power application. You'll likely need proof of being in Cambodia such as entry stamp and hotel booking/address.