Usually my mental model is can consider buying miles when 1 mile cost $0.015. Take for example 37,000 miles is a 1 way business class to Cheng Du or Guang Zhou. Return is 74,000 miles ($1110). The rest cost of a business class ticket from SG to Guangzhou is $2,500. So you save ~ $1,400. First class you probably save more
My wife and I only use 1 main credit card. We have another 1 credit card as backup. The 2nd credit card is also a card my wife gave my parents to use.
Because we only have 1 card, bank always give us waiver. The 2nd card most of the time also got waived. The 2nd card typically didn’t spend much but still got free waiver. Maybe is because my wife has some investment with the banks so they close 1 eye.