I want to go on a cheap month-long adventure....

[StarlightWonderStarlightBright]

with ds (3 and asd) and dd (who will be 20 months).

We don't want anything too commercial or expensive, just an opportunity to do new things, find ourselves and spend quality time with the kids.

Any suggestions?

DH is coming too of course!

I think you are mad , you need to ask yourself who is going to benefit from the holiday. We lived in Thailand for four years and I've lost count of the number of adults dragging bored, distressed and sweaty children around while the grown ups found and centred themselves.
Have you considered a kibbutz. They were all the rage when I was young (a long time ago), you'd be doing something worthwhile too.