What I found were some data pages from a Chulalongkorn university study, which listed data on elderly people from the censuses, including the total population. For example with the page on Surat Thani - one of the provinces where I did not have the exact data before - showed the total population in the 1960 census was 324,784, and grew to 747,049 in 1990. The more recent datapoints were then taken from the registration data. Once I filled all the holes, the sum however did not match with the total number, which took some time to finally spot those provinces where I had entered the number wrongly in past - or where the PDF had a mistake, I did not verify back with that one.
Just one thing did not add up correctly - 1960 and 1960 there were still the two provinces Phra Nakhon and Thonburi, which were merged into the special administrative area Bangkok in the early 1970s. The Chulalongkorn data had separate data for Thonburi and Phra Nakhon, but the numbers don't add to the value listed at the NSO as the regional sum.
For the older censuses I wasn't able to find the data for each province - while Statoids has numbers listed, the sum 17,256,840 given there is not the same as found in the NSO table, according to that the total population in the 1947 census was 17,442,689.