I think Biman air still flies from BKK to Dhaka. Try them for a price.
And just a note, if you fly to Nepal and drive through India to get to Bangladesh, you cannot re-enter India for 2 months. So be sure your Bangladeshi visa will allow you to stay there for at least the 2 months. Also, the re-entry into india from Bangladesh when you want to leave may not be as easy as expected. ie. you may not get your 180 days. It will all be up to the guys at the border.
Just a heads up
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