Thai airways flight- United V/s Thai

[blacksultan]

I needed to book a BKK- MEL flight later this month. Availability showing on United for this Thai air flight.
I can`t check thru Royal orchid as I have 0 Miles in my account.
Was going to transfer from Gateway to SPG to Marriott to United.
Is there any reason I should not do this and transfer directly to ROP and book through Thai?
It seems the no of Gateway miles required for both are the same.
Are the taxes likely to be the same?
How long does it take to transfer from Marriott to United?

 

[blacksultan]

Also do have Thai and United have the exact same award inventory?