Raspberry Pi UPS with 18650 + TP4056 + MT3608
You are using IRFP9240. This is not suitable MOSFET for this task. This MOSFET requires higher Vgs than you are providing. You are going to have Vgs around 3.6-4.2V. IRFP9240 requires around 10V to fully open (or at least 5V to be usably open).
Try different MOSFET with much lower Vgs(th). For example something like PMV48XP (I just randomly googled, you can find plenty of suitable MOSFETs).