There are a lot of generalisations in your post.
Not all teachers wish to teach "where the action is". Therefore, schools do not know this.
Agencies are not a bad thing. There are 2 very good agencies that operate in Isaan. Lesson plans provided, test papers provided, co ordinators on site at all times, rising salaries (40k possible within 2 years), visa, extension and work permit sorted and paid for.
30k is not old news everywhere in Isaan. Most schools outside of the larger towns will not be able to afford anymore. 40k is doable, as explained above and as you seem to be earning, but not a given. Especially for a new guy in a new job.
Not all towns in Isaan can offer jobs where some sort of transportation is not required. Whether it is a bus, train, motorcycle, taxi, tuk tuk or car. There is a lot of difference in each province. Town centre of Khon Kaen is a lot different to the next district even though it could only be 10kms down the road.
Admins of schools also vary. I've been in BKK and worked for very good schools with good admin departments. I've also worked in Isaan towns where the admin are sh%$ and do give much notice to foreign staff.
You seem to have a good job and one that gives you what you need. That does not, however, mean that it is the same everywhere else.
Cost of living??? Varies also.