I ended up splitting my time at 4 places, all in/around Playa del Carmen -

Westin Cozumel - beautiful views, this place was amazing. The platinum breakfast was nice, pastries, fruit plate, juice & coffee, and a hot entree of your choice. Getting there isn't great – arriving to Cozumel was total hell on a dangerously overcrowded ferry, and I didn't care enough about the town to even leave the hotel once. The restaurant for dinners was decent, not the world's best cuisine but just fine. It was perfect to just sit with an ocean view and work. Very pleased. I did get eaten alive by bugs.

La Semilla - I tried a night here, also got more mosquito bites, all the reviews are very positive and this place is cute and charming but it's a very very small b&b and I just didn't find it comfortable enough to spend any amount of time in, especially if you need a workspace. There's a cook who makes breakfast in a beautiful courtyard but I found this breakfast to be very fattening (tons of crema/cheese), I was too easygoing when I ordered and probably should have been pickier.

also non Marriotts -

Fives Downtown - this is a Hilton, I have Gold status with them - and I got a nicely upgraded room, the comp breakfast in the restaurant was delicious, probably the best of all of them, and the rooftop pool is a splendid place to lounge with a laptop, and it was only $95/nite. A weirdo or two were staying here but the staff was amazing. For $95/nite it was a great bargain. Only drawback is the rooms don't really look at anything, just an interior courtyard.

Thompson Main House - no Hyatt status, I just thought it looked nice, and it is very very nice. They do have an Amex Platinum f&b credit offer for 2 nights+, or a good halfprice f&b offer ($50 for $100 in f&b credit). Very stylish, the rooftop pool is a super loud dayclub/nightclub, not really "WFH" material but you can also access the Beach House, rates there are 2x so I cheaped out and stayed in the Main building instead. The Beach House is splendid, an amazing place to sit with laptop. And if you get a room w/a balcony - also great place to work from, even if you hear the vibrant noise of 5th ave.

If I can get away again, I'll try some of the Merida area haciendas. Thanks for all the advice.