Until recently, maybe 4 years ago or so, a comprehensive road death tally. was reported on a daily basis on this forum during these holiday periods.
Those reports, in tabular form should the 'deaths on the day' as reported by the police and it also reported subsequent deaths as reported by hospitals. Those subsequent deaths were added to the totals as and when they were received by the compilers. So it was quite clear that they do not only report 'deaths at the scene'.
Finding a link to those reports would be a mammoth task now and I'm not even going to bother trying. I'll leave it to Googles AI to do the heavy work.
'Thailand's road death statistics are collected from various sources and compiled by the Department of Disease Control of the Ministry of Public Health. Data is gathered from death certificates, Police Information Systems reporting traffic injuries, and central motor vehicle insurance registries. This integrated approach, supported by ThaiHealth and WHO in Thailand, aims to provide a comprehensive overview of road traffic deaths'.