Using Singapore EP to register Futu and Tiger to trade US stocks, will you be taxed by mainland China?

Actually, it mainly depends on which country you are a tax resident of.
If you are a Singapore tax resident, you won't be checked.
But if you stay in Singapore less than 183 days a year, your Singapore tax residency might be canceled, and the information could be sent back to mainland China.
Additionally, there's the issue of fund flow. If your trading funds are directly transferred back to a mainland Chinese bank, you could also be subject to investigation.