5 volt small UPS design (review request)
The TP4056 module is good for charging but it's quite possible the step up module has a 20mAh or so quiescent current and it might drain the battery even when it's not powering something. If you're not using this for a very low power device then it might not matter.

In my opinion, a better option would be to use something based on TPS61090 such as the PowerBoost board.

Otherwise your concept it's clearly cheaper if that is more important here.