How does 周到 mean "thoughtful"?
Sorry I don't know this word's etymology. But maybe understand the synonym helps to remember.
As far as I know, 周到 has nearly the same meaning as 面面俱到（miàn miàn jù dào）.
周 (zhōu) is like 面面（miàn miàn）, meaning in almost all aspects; and 到 (​dào) means taking care of it. So that 周到 means taking care of all aspects, that is thoughtful.