Bonjour Michael !
I agree that it's indeed quite a tricky point!
Indeed, after looking into it thoroughly, I can't really come up with a better explanation than "colloquial usage", which I admit is not really satisfying :)
I would say however that we will tend to use soi rather than soi-même after prepositions (devant, avec...) and with comparisons or after que in general.
Soi-même will more often be used to emphasise that you do the action yourself:
Il faut le faire soi-même.
I hope that's helpful!
Merci et bonne journée !
