My 2 cents..

With around 90 minutes to spare I'd consider going straight to the south lounge complex to try and book a spa treatment.

Drop in the South lounge for a drink or few, then ask at the desk in the lounge if they have a preliminary gate for your flight. If they say it's a T5B gate then head over there to the relative calm of the T5B lounge. There's also a travel spa over there that can have more availability than the main one.