Focus on detailing his work online, business registration, screenshots of websites, LinkedIn profile etc. Letter detailing permission to work remotely in Thailand and with no Thai clients.
You should know business class flights, 5-star hotel bookings are not mandatory requirements for entry to Thailand, is the point. I do not think i know better, im just giving you a fact about entry.
You really think a business class ticket is required for a valid visa entrant along with a 5 star hotel? How often do you open your passport and visa to strangers on arrival and question them on the visa application? How can you hear multiple people being questioned on arrival? I have given a couple of my entry experiences, once asked what my job is that's it. Majority of time is nothing or questions on English football teams
Posts that said business class tickets, expensive hotel bookings help entry are BS. Saying" I saw many arrivals passport info and visas and heard everything what the IO said to them" is BS. The poster often removed the post quickly when the lies are called out