Welcome to Thailand FT.

As far as I am aware there are no lounges in the airside arrival area – that said, living in BKK I don’t have the need to use a lounge upon arrival so never looked for one. Maybe someone else can advise.

Regarding left luggage facilities there are 2, but both are located on the public side of the airport. For details look at http://www.suvarnabhumiairport.com/e...orage-services
Regarding picking up your luggage at BKK much will depend upon your ticketing.

If you have a single ticket IST-BKK-USM, then it possible that the check-in agent at IST will check your bag(s) straight through to USM and not allow you to collect a bag(s) at BKK. My understanding is that this is due to rules relating to fare pricing.

If this is the case you will need to pack warm weather clothing in your carry-on and change on route to BKK. On disembarking follow the normal arrival route via immigration/customs and leave your carry-on bag at left luggage on the passenger terminal side and then proceed to TG check-in.
If you don’t want to leave your carry-on bag you could transfer from your TK flight to TG airside. See details: http://www.suvarnabhumiairport.com/e...-boarding-pass

If you have 2 separate tickets e.g. IST-BKK & BKK-USM, then follow the arrivals sign to immigration, collect your bag(s), clear customs and exit into the passenger terminal. The airport is very large so it will be a matter of finding somewhere quiet to transfer cloths, leave your bag at left luggage and then proceed to check-in at the TG counter.

Regarding ‘Fast-track’. As a TK J pax I’m fairly sure that you will be given a priority pass valid for normal immigration on your arrival. Yes it is much quicker (5-10 minutes) than the normal plebs lanes which can take an hour at busy times. I don’t think that they have an immigration Fast Track lane in the transfer area, but queues are reported as being normally short. TG J pax have their own security check lanes which again are fairly fast.

With 6 hours layover you really will not have any problems whichever way you come through BKK.

Hope this helps and have a good trip.