I'm trying to be pragmatic about designing a PCB and circuit that will have to dissipate heat. It's pretty simple: a surface-mount power transistor charges up a 4F 5v supercapacitor with a roughly constant current of 2A. It will not do this repeatedly - so it only needs to dissipate about 20 joules over 10 seconds, once, from room temperature.
I like the idea of killing two birds with one stone: keep calculations simple by building in safety margins and taking worst-case simplifications. But calculations or thermal dissipation over time, especially combined with electrical dynamics is not simple, mainly since the PCB's copper tracks and their layout have such a significant effect, but also since the transistor is not ideal.
So I imagine the only two practical ways of designing this are one or both of: with specialist CAD software, or testing of a physical prototype, simply measuring the temperature parts of the PCB and its components reach.
Realistically, what would be the simplest, effective way of doing the electrical and thermal calculations/assessments necessary?