Koh Samui to Koh Phayam in a day (July)

Go to the proper bus station in Nathan and buy a ticket to Ranong. Local and not tourist buses.

It depends on how late the flight is as to whether it is worth trying to do in one day and choice between either overnighting in Ranong or Kawthoung would be based on similar criteria but I would err towards the latter

There are some direct mini buses from Surat to Ranong . You could take the night boat from Samui to Surathani arriving in the early hours and then get the next mini bus to Ranong or if necessary take a mini bus part of the way and change to another if there is not a direct service to Ranong at the time you need it.

No ! this is not good time to go to Ko Payam (Phayam) if seas are rough boats may not be running or if they are you might get stranded there if the boats can't leave the island if the weather turns bad . Sometimes the beaches on that island as well as Little Ko Chang can get engulfed by violent seas at this time of year , plus there is also likely to be heavy pelting on and off again rain.
Stay in Ranong - try Wood Guesthouse in the market area. It would be nicer to stay there than Victoria Point (Kawthoung). You are allowed to enter Burma that way via Victoria Point and continue to Rangoon with tourist visa ? ie - its an approved entry point ?

Flight Kawthaung to Yangon is about how much? 160$ one way?

Considering that you don't have much time, are not planning to see anything in that corner of Myanmar and that Phayam/Ranong is one of the wettest corners during the rainy season and the seas can be rough and boats delayed I think it would be wiser to skip that detour and fly Suratthani-Bangkok-Yangon.
Check out Nok air, should get you there for 120-130$ one way, total and gets you there the same day!
Saves you the long bus ride too.

Thanks, all - very helpful and I think just spending the night in Ranong makes sense, thanks for talking me out of Phayam! From memory the flight was late afternoon but I've just checked again and it's 2pm so that'll be difficult in a day. Yes, we have visas and are allowed to cross at Kawthoung.

Tools, I get your logic but I quite enjoy land border crossings - masochistic I know!

I do too, but only if I have the possibility to spend some time along the way.
If you go that way hop over and spend the night in Kawthaung, walk around there, spend the bit of time there.
Walk through Myoma market and take the small alleys and stairs up to the pagoda on the hill to the right, overlooking the town. Go down the backside and wander through the narrow roads there by the sea where all the fishing boats are anchored.

Victoria Point nothing to write home about and if you have to take expensive flight to Rangoon would also question whether its worth it as Tool4Fools said. Yes its allowed to cross into Victoria point for day visits , but you are sure you are allowed to travel further into the country from there ?

@davelliot As sure as I can be, based on the latest edition of LP Myanmar, and forum threads/trip reports here and elsewhere, but thank you for dishing out the usual TT-style disapproval.

Thanks Tools, good recommendations.

@tinydog ,
No, did not mean to dish out "TT-style disapproval." , was just curious . But again the weather could be a problem , Victoria Point in itself is not anything special and if the flight to Rangoon is expensive I share Tools4fools reservations about the plan .The journey from Surathani to Ranong often has some superb scenery along the way if its not raining however.
Would also add that Payam / Little Ko Chang is not just one of the wetter places in Thailand during rainy season there has been known to be very bad weather there sometimes even in December.