I have a 2-bedroom 55 sq. m. apartment in BKK with nice gym, club house and 50m infinity pool and the rent I saw at the real estate agent downstairs is only around $430-500 (this is post-covid. Pre-covid, a friend of mine rented a room this size next to my room and it cost him $570-700). Fortunately for tourists, BKK has an oversupply of apartments or what we normally call "condominiums" (apartment for us is more like a no-frill studio room without any amenities like gym, pool etc and it is much cheaper than condo that's for sure) and that makes the rental price very competitive and it got even cheaper due to covid. I've compared notes with some people here regarding the depto selling price and it seems like it's relatively similar to the price in BKK but it all depends on what barrio I suppose. Mind you, BKK has some very fancy condos like a small room sized 30 sq. m. could cost $200,000+ depending on the area. Normally the expensive ones are within the walking distance to the skytrain (BTS) and subway (MRT) or by the river.
About locals not cooking, most of us don't cook anyway. It costs only $1-2 for a meal so it's cheaper and more convenient to eat out or buy something to go. I cook because I love cooking and I only cooked on the weekend when I was living in BKK. Weekdays after work I just bought dinner on the way home from my office. I always saw a good mix of locals and foreigners at Terminal 21 food court and it was always busy and a challenge to find a seat and since it's so cheap, I suppose it attracts local tourists too. I did go there to eat sometimes if I happened to pass through the area.
If you stay at a nice hotel like 4* up like the Hyatt, breakfast is always amazing. They give you a variety of local and western choices plus Chinese and Japanese options. I went to Iguazu last month and the hotel was nice too (Mercure) but the breakfast was just OK. I find breakfast at hotels in Argentina heavy on sweet baked goods like 80% on those and the rest is hot food and others. I'm more of a hot food kind of guy when it comes to breakfast.
The heat is BKK can be brutal. You get used to it or you can manage it if you live there. That's why everywhere is airconditioned and Thais like to go to shopping malls as it's nice and cool. Only Europeans stay out in the sun or tan themselves which is understandable especially if they come from Scandinavia where it's always dark in the winter. We don't do that and we try to stay away from the sun as much as we can. We use umbrellas to walk in the sun.
The traffic can be very bad and it has always been that way for a long time. But now there are skytrain, subway and airport rail link so I normally utilize all those to avoid traffic. And if I need to go somewhere really quickly during rush hour, we have motorcycle taxi for that
Sukhumvit road is probably the most traffic congested road in BKK and that's where you stayed. That's why you were stuck in traffic. I'd never drive on Sukhumvit since the BTS runs through the entire road. I suggest only use the driver when you go outside of the city center. It's faster and easier to go around with BTS and MRT.