Krabi to Phuket will take about 2.5 to 3 hours and with own driver and aircon vehicle will cost about 2,ooo bhat.
Only Bangkok airways fly in to Samui and out.
I don't know much about the north but someone else will advise you.
You are travelling in Peak season so prices will be higher and transport fuller.
i don't know about to Aus regaring different setting off and landing points.
don't put too much in that you don't enjoy it. Don't forget that Samui is right over the other side with a different weather system etc. It may be that there is still a bit of rain if you go late December very early jan.
Enjoy I like the prep as much as the going sometimes.
Don't do everything you will be back.

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Kiss
God laughs at people who make plans.