How to calculate maximum power supply rail current (Maximum load current of a system) based on datasheet current values?
I think you're overthinking this way too much.

1) what are you hoping to gain by designing the DCDC converter to exactly match the current consumption of the system?

2) If you dimension the DCDC converter such that it cannot deliver the current required you risk system instability (it might reset). So the safest choice in my opinion is to use the Maximum current and add some margin. So here that's 230 mA with some margin so 300 - 400 mA. From my experience, that number sounds about right for a BT + GPS system.

3) The system's current consumption is NOT constant, it will vary with activity. When everything is active it will consume more current compared to an idle situation. The software in the microcontroller has a huge influence on current (power) consumption. Imagine the difference between updating the location every second or once per hour.

4) Since power consumption varies so much, for battery powered applications it can be useful to select a DCDC converter that has an efficient low-current mode. Some DCDC converters use a PWM mode for high currents and a PFM mode low currents. In PWM it switches continously, in PFM it switches in short bursts.

5) If you are not concerned with efficiency you could use almost any DCDC converter capable of delivering at least 300 mA.

6) Only after you have your system up and running will you be able to measure the actual current consumption and then you might be able to optimize the value of the inductor in the DCDC converter to gain a little bit
of efficiency.

Oh a, final tip: DCDC converters can be tricky beasts. There is no shame in just using a ready-made module. There are cheap ones from China but also direct drop-in replacement modules (for the 78xx series of linear regulators) from reputable manufacturers. That will save you the hassle of trying to get the DCDC converter working when it doesn't work for some reason. In some teardowns of very expensive (professional) equipment I have seen ready-made DCDC modules being used, the designers of that equipment wanted something that "just works" so they used a module designed by DCDC experts. It might be a little more expensive but when in a hurry, it might save you b#tt!