You are likely going to have to fly in and out of Bangkok, so do the Murray Head/Chess thing. (One Night in Bangkok) Ride the on/off tourist boat up the river and spend the day seeing the sites accessible from the boat. (Palace, major temples, etc.) At the last stop, walk over to the Khaosan Rd area and get dinner at a sidewalk restaurant, followed by an Uber home. (Uber drivers are honest, you know the price up front and don't have to deal with the issues mentioned above.)

You do not need a tour guide for this. Ahead of time find self guided tours to load on your phone and use those at the major sites. Do not talk to the people who stop you on the street. Just don't engage! The palace is not closed on the day you are there, there isn't a special entrance down the road through a silk shop, etc.

You mentioned Marriott... If you have status, both the Marriott Soi 57 and the Marriott Marquis are very generous with suite upgrades and have fantastic Concierge Lounges. The Marquis has the nicer lounge (perhaps the best of any Marriott hotel in the world) and the Soi 57 great views. These hotels are actually better than the JW or Ren. If you are looking for a bargain, the Courtyard is nicer than what you find in the US.