Lounge Access For Thai Airways & Business Class Upgrade

Good Afternoon Forum Members,
I am taking my family for their first overseas trip to Japan. We are travelling from Melbourne using Thai Airways (class Q departing and class V returning) to Toyko with a stopover in Bangkok both directions.
My question is regarding airline lounge access and the possibility of upgrading our tickets to business class. When arriving in Bangkok, can you pay for Thai Airways lounge access or is it only limited to the upper end of town? And, would it be a waste of money to pay for the upgrade to business on check-in? (if that is at all possible).
Discussions at work mentioned if I had a Gold Star Alliance membership that I would be able to upgrade my flights to business class and this would also get me and the family into the airline lounge. Is this really the case?
My apologies in advance for my inexperience in this area and hoping someone from the forum might be able to point me in the right direction.
Kind Regards and thanks in advance for any advice.

At times, it has been reported that TG does offer upgrades for sale at check-in. If they have them available - and that's an "if" that may or may not happen - then they are generally at a good price so well worthwhile. And yes, that would also get you lounge access. As far as I know, TG doesn't sell lounge access at BKK, though I believe there is another lounge where you can purchase entry. It is the Louis Tavern CIP Lounge. (You can also access the King Power Lounge by joining their duty free club at the entry.)