I fly domestically a lot and prefer Bangkok Airways, with Thai Smile being my second choice. Neither have been concerned about the weight of my carry on (many low cost carriers weigh carry on bags and charge if it's overweight). The free checked baggage allowance is 20kg per person. You can find their full baggage rules here if you have excess baggage.

Although Krabi/BKK is only about an hour in the air, they do serve a meal/snack. There is decent leg room that is ok for such a short flight. You will select seats at the time of booking (even before paying with your credit card), but check again 72 and 48 hours before the flight and you'll find additional blocked seats at the front will open.

As a full service carrier, I've never found there to be much of a queue at check in. Since it's a domestic flight you can get your boarding pass on the app (fi you didn't have a bag you'd go straight to the gate). At the check in counter look for the special queue for those who checked in online and drop off your bag.

They have been very accommodating when they changed a flight time. I called about being rebooked on a different flight and once when other flights didn't work, they quickly refunded my money to my credit card. The miles are so insignificant it probably doesn't matter if you sign up for their frequent flyer program or not.

Like you, I fly business class for longer trips, but flying Bangkok Airways economy for short domestic flights is perfectly acceptable