I arrive into BKK at 01.30 and want to head up into Issan ASAP.

If I go straight to Morchit are there any buses leaving at around 02.30AM?

All the websites I can access say they start around 06.30.......

There is a bus that goes from the airport bus station to there but dont know the times sorry

Yeah, I believe there is a better chance that there are buses outgoing to Isaan {and other} destinations at 2 AM - 3 AM. Regardless of the online findings, which I'm guessing are outdated and not correct, Morchit is quite the busy terminal around-the-clock.

I mostly fly in to SV at around midnight . I came in on a delayed flight to SV one time and missed my Nakonair last bus up to Ubon leaving about 0100 am . I took a taxi up to Mochit any way and got on an aircon bus at about 0345 am. This bus got me into Ubon at around1600 pm the next day as it was a stopper . I still had a fair way to travel . I would say wait at the airport and get the first domestic flight up country in the morning . Or you can get a taxi . I took a taxi up for 6000 once it was about 6-7 hours . I just sit it out at the airpport these days .

Let us know if you find a good route .

cheers and good luck .