1. Do you think you can hire a driver in downtown Bangkok and just go anywhere you want to go? Did you ever consider the maddening traffic and how it can take at least 1 hour to move 1 km?
2. The cost of living in any major city downtown is going to be much, much higher. Where 170,000 is great in Isaan, 170,000 in the heart of Bangkok, to do what you want to do, is not enough for the things you claim. Those that think it is enough are relating it to living in a 1 room flat and eating off the street. There are many restaurants where you can spend 3000 thb per person, if you want to be a big spender.
3. Do you ever check the air quality in Bangkok? Do you really think an air purifier in your place is going to save you? Come on man. Bangkok was built for 1,000,000 cars and there are currently over 12,000,000+. In the past 10 years, the air quality is Bangkok is super poor and getting worse. I would never consider living there for that reason because I don't want to die early. Do some research of your own. Air quality index so far off the charts it is maddening. One walk down the street and you will be hacking and coughing for days.
4. Hospitals, medical care, dental. All much higher in the center of Bangkok.
5. Gyms and teachers of course are going to charge more, their cost of living is high as well compared to areas outside of the city.
6. Your claim to want to see temples? What happens when you see all the temples around Bangkok in one week?
7. Bangkok is one giant concrete palace. That means it is much hotter than anywhere else. Walk 5 minutes down the street and your shirt will be drenched in sweat.
8. Crime and scams much higher in Bangkok.
Although there are some individuals with unique attitudes on TV, one thing is for sure. Many of these folks have LIVED in Thailand for years and they know what they are talking about. Those that have never LIVED in Thailand and only visited are in for a rude surprise.
If you were smart, rent a place for 2 months in the heart of Bangkok. You will learn all you need to know before committing long term.