I've taught in both the UK and the Thai education system. I too managed to 'almost' have control of my classes in Thailand by having fun activities and providing rewards and even old fashioned punishments like detention (agreed with parents).
Being alone with 35-37 eight year olds in a Thai school is a huge challenge of your sanity. No matter what you do, you'll find that the pupils show more respect towards the Thai guy fixing the aircon than the 'farang' teacher. I've seen Thai kids fight, heard them swear in English (quite a lot) and refuse to stop talking to their friends during a reading exercise (teaching reading to Thai kids can be a nightmare if you have a large group of low level pupils).
The reason for all of this is not always the teacher's fault. Nor is it really the pupils' either. It's the system as a whole.
Let's be honest. Thai kids have more 'respect' for Thai teachers for two basic reasons.
1. They speak the same language and can reason with their pupils.
2. Thai teachers tend to have big sticks they beat the kids with for as much as rocking in their chairs. We've all seen this.
Number (2), in my opinion, undermines any real hope of a western teacher controlling a large class of primary pupils if teaching alone. The pupils only react to this negative reinforcement. If they don't fear you, it's time to go nuts and do as they wish.
In my time teaching in Thai schools in the Thai system, I could manage to control most of my classes, but some were simply impossible. Sure, countdowns work for about 1 second. TPR can last a minute or two (if alone with very large groups).
Some subjects are easier than others to maintain control (Phonics/general English). Reading and writing or academic writing can be an absolute nightmare if you teach in a school with kids who only get 2-3 hours per week of English tuition. I've worked in schools that gave the kids books that were far beyond their capabilities. Books that merge several tenses, regular and irregular verbs and about 30 new vocabulary words for 7 year old Thai children. And the school gave us 50 minutes to get them to read 250 words and answer 3 pages of comprehension questions. All this, when the brightest pupils in the class are still getting to grips with using regular verbs in the present simple ("She like dog.") Yes, I suggested they change the books. I might as well have asked the cat.
The system is a mess, to be honest. If you see a job with classes of over 35 primary aged pupils with no assistant available...run away! Particularly if you'll be using books way beyond the abilities of the pupils.
On a positive note, you can make a difference if you 'hang in there' and become creative with your lessons. But if you're forced to do 'pages 42 -45' of the reading and writing book today, when this book is clearly aimed at native English speaking pupils and not your 3 hours per week mob...don't look back!