of course there is an option to buy business class seats with Etihad - even just looked at the date you mentioned and there are business class seats for sale on that date.
When are you flying back? Are you sure you still need a test? If you do I used Dr Donna at Medconsult. I did the antigen test for 900 baht at the time - think it’s now 600 baht - this test is good for UK entry.