Good place to work remotely and enjoy the tropical rainforest

I think your requirement may be a bit generic to get any detailed recommendations. If the tropical forest is secondary to working online then you would have to stick with the major parks as they only really have the external infrastructure to support you. The time of year would be critical to know as well as that can affect whats available. 

I don't think you will find a professional guide at 1500 baht (45 USD) a day (not in Central Thailand anyway), but you used to be able to approach the rangers in some of the parks for trips, but that would not be viable unless you speak Thai. I have noticed that making arrangements with rangers has completely stopped in some parks though so not sure if this still viable. These were always ad hoc and could not be made in advance as the rangers don't know what their weekly assignments are. And of course no English and the rangers don't always know a lot :+)

Multi-day treks within the borders of a national park. Very difficult IMHO. If you read through the other post you will see even people living here have difficulty tracking down people to arrange these.

What would I recommend, well I am probably not too experienced in this type of recommendation but you could try to dig deeper on: Khao Yai (lots of local infrastructure, bags of animals, great forest, reputable and experienced guides); Khao Luang (you could stay on the coast and do Khao Luang plus if you search theres some details on here about a potential multiday within the park); The North, anywhere in Chiang Rai Province - no megafauna but seriously beautiful high elevation forest, and lots of local support, lots of multi day treks by locals in places like Pai); Kaeng Krachan/Kui Buri lots of animals, megafauna, dont think multi-day possible though). 

If you wanted really good forest with masses of wildlife and great external infrastructure and accommodation opportunities then you should consider Khao Yai seriously.

Going with projects/researchers. Not viable IMHO. Researchers/Projects work under very strict MOUs with the DNP. I have had a project permit for quite a few years and the admin, bureaucracy and control around them would not make me risk taking anyone not cleared under the project documents or parks's approval under any circumstances.

The north may suit your requirements better, lovely forests and biodiversity, not many mammals buts lots of birds, trails, etc. 
 

As Paul recommended I think Khao Yai is one of the best spots to just visit on your own. One of the best trail networks in the country, even though these days fewer trails are open to the public than in the past. Perhaps nice to do a guided tour at the beginning of your trip to get to know the place/animals a bit. But then you could just rent a car/motorbike and enter the park by yourself.

Kaeng Krachan is great too. Check out Baan Maka nature lodge if you are looking for a place to stay for a long period. The lodge grounds will give you a jungle feeling. You will have Oriental Pied Hornbills flying around your bungalow, and a wide variety of other birds, small mammals, and herpetofauna on the lodge's premises. And a constant stream of naturalists staying there, so a great place to meet like-minded people and get tips where to go. To my knowledge, it is the best lodge in the country for this specific purpose.
But you would need a car to visit the park, in Kaeng Krachan they do not allow motorbikes (except the staff). And best to rent a 4x4 vehicle so you can also visit the higher elevation area.