FYI – The flight from HKT-BKK is internal, so there will be no need to clear immigration upon arrival. I would guess about 30 to 45 minutes from deplane to walking out into the arrival area max.

Most airlines don’t open their check-in much before 3 hours prior to departure, so if you catch the earlier flight you’ll be left hanging around with your bags until check-in opens and then a further 3 hours until departure. Boring

Although you said that your flights are on separate tickets, you could ask the check-in desk clerk at HKT if it would be possible to have your bags checked through to your next destination. It will depend if TG has an interline agreement with KAL and the mood of the clerk/supervisor. Nothing to lose, so worth asking.

If you decide to take the earlier flight and have to collect your bags there is a left luggage facility at the airport if you want to spend a few hours in town.

Personally, I would take the later flight. You will have plenty of time to collect your bags (if necessary), check-in and clear immigration without any rush.