In the application state how long you have been in a relationship, put copies of your own bank statement in showing liquidity (upload the bank statement pdf). Hand write a letter stating you are paying for the whole trip and estimated dates of arrival and departure from the UK , sign it , get a copy of it.
500 a day max 20k . If you get stopped before you get to the airport such as a road checkpoint you will be arrested and it’s an automatic 3yr ban even for 1 day overstay
with 2 METV in your passport you will probably need a proper long term visa. Living in Thailand full time on visa exempt or tourist visas is eventually not going to work and you will be denied entry by air or land
Enter on used Thai passport. Leave Aus on Aus passport. If entering on Aus passport will get a 30 day visa exempt stamp and then things get complicated as would have to leave Thailand within 30 days to close the stamp