You must start with cash in a Thai bank then go on to one of the authrorised platorms I believe for the zero tax regime in place ?

I have crypto offshore on a non-compliant platform, as well as cash, and so I'm guessing I need to off-ramp in my country then wire to Thailand and then on-ramp to bitkub etc ?

Would that be right ?

I know there may be grey zone ideas, but I thinking forward on any papertrail issues to be 100% straight down the line.

You don’t need to do that, you can sell direct on (say) BitKub.

Tax is due on the amount and source of funds at the tax point you bought your crypto.

For example

You bought for $100 and sold crypto now for $1,000. The $900 capital gain is tax free, the $100 initial stake is subject to normal tax rules so it depends when you bought & source of funds potentially.