I think you have enough time. Takes about 1hr 30mins to get to bkk from dmk that’s if you know exactly how to get there by train without getting sidetrack. It’s actually not that hard. Take the bts to Phaya Thai then airport rail link to bkk.
I’ve gone at 0800 and they open at 0900 two times with different wait time. First time I wait about 1.5hrs and second time about 6hr that’s including them closing for lunch at 1200-1300. Both times was to get an 30 days extension. It’s really a hit or miss on how long you wait but going early would be better than afternoon.
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