seems like it's hard to get short term rentals in this price in the areas I mentioned. Now I'm looking around Phra Khanong and On nut. (nearby BTS). Might have to increase the budget to $600-$800

Is that your budget for just your rent? Or for all your living expenses?

The Hive is a pretty bad coworking space. Low ceilings, cramped, and bad value for money. Recommend Paper Plane Project (free) or True Digital Park (not free, but very good quality chairs and nice space) instead.

16K thb for rent won't get you anything close to the BTS in Phrom Phong, Thong Lo and you will want to be close to public transport especially on a 2-3 month trip. Also beacuse your time in the country is short, expect to pay a lot of money for rent (2-3x the normal rate) - landlords give the best deals to people who sign for a year, not 2-3 months.

Ultimately, better to look in Phra Khanong, Ekkamai, On Nut with that budget - you'll get more bang for your buck. I like Phra Khanong the best out of all of those neighborhoods, it has a local feel and some very cool markets + a nice nightlife/food area called W District.

Just go on Fazwaz/propertyhub, find buildings you like, then visit in person and ask to work with the owner directly. Don't use agents or if you must, ask the building for their preferred agent to avoid getting scammed. Generally though, agents take a fee from the owner, so you won't get the best deal doing that (owners are less likely to negotiate with you if they're already eating a fee from the agent)