Using TP4056 module to charge battery while load remains connected
Looking at the schematic of your TP4056 board



you can simply add a pair of diodes of low forward voltage diodes to achieve what you want. SS14 are common and very cheap. Add one diode between the TP4056 IN+ and MT3608 VIN+ and one more between TP4056 OUT+ and MT3608 VIN+.

Your MT3608 will then pull power from USB when there is USB as 5V will always be larger than the 18650 battery voltage.