My two pennies' worth for what it's worth...
I built a fairly large detached house in Isaan on a good size plot in Sakhon Nakon in about 2007 and needed to sell it in 2015 (slight correction there, my friendly and wonderful value for money Thai builders built if for me!)
I found local Thai property agents more than useless - zero viewings came from them and I couldn't see any evidence of effective marketing on their part.
In the end we just put it on the Thai equivalent of Craigslist - can't remember the name of it now, but maybe Bahtsold or equivalent.
Some people who owned a coffee shop in our town came to see it after the online ad had been up for a while (that's where they'd seen it) - they were surprised to see me at the house as I was a regular customer at their coffee shop!
They needed a larger house for their extended family and liked the look (and price) of our place.
They were cash buyers - we agreed a (low in my opinion) price compared to the asking price, however I was keen to sell and because of the exchange rate differential in our favour (baht was very cheap when I transferred money in from abroad and expensive when I sold the house), it actually ended up being cost neutral on the whole and I lived there essentially free for 10 years, so I didn't have much to grumble about.
Overall my impressions of the Thai property market were:
It's not nearly as developed and efficient as in the West
The process is quite nerve-wracking, you start to doubt you're ever going to sell it
You need to be very flexible on price
The percentage of people who can get access to 2-5 million baht (or whatever your price is) is vanishingly small compared to Western countries where many more people can get hold of mortgages etc.
The plot-size/land/location value is key - the possibly well-built house that's stitting on it doesn't seem to factor into the equation very much and may even be considered "2nd hand" as opposed to say the UK where Georgian or Victorian properties trade money for big amounts. I guess that's partly a factor of the very relaxed urban planning system in Thailand compared to the UK
As other posters have said - it's likely the land value that will drive the bank valuation, other considerations appear secondary.
So - good luck, keep plugging away and as others have said, consider your own digital marketing campaign and see what kind of bites you get.
As with any commodity, it's only worth what someone is willing to pay for it, and that market demographic in Thailand is quite small, although there may be more money sloshing around there now compared to 10 years ago.
If I ever live in Thailand again, I would look to rent long term in the first instance, and I would only consider building something in the max ~1Million baht range; very simple bungalow with tiled floors type thing, something I could walk away from without too much pain if necessary.