I haven't been there yet. People are not talking good about Sihanoukville (anymore). Kep seems to be the better choice. While I was forced to do border bounces, I've gone to Koh Kong several times. Not bad and easy to reach by bus to trat plus van to the Checkpoint. Always good for a couple of days.
since Phnom Penh is a shithole, I recommend a flight to Siem Reap. Visit Angkor Wat. If it's about a border bounce for another 60 day stamp, you can go by van to Ban Pakkad. Start is 6.00am, back in Pattaya about 3.00pm.
1st time in person. Ask your landlord first if he filed already the TM30. If he didn't, be prepared to pay his 1600,- fine at Immigration. After that you may report online.