Hello Siebe,
“Why would the Buddha suggest to make an island or refuge of ourselves when we would only be fleeting processes……,”
You mean, Siebe, how could the Buddha advise taking refuge in the fleeting? I take this to mean: rely only on your own efforts (not even He could attain anything for any one else). And what effort would that be?: “I take refuge in the Buddha…the Dhamma…the Sangha”.
Now if there is something that is forever unfleeting it is the Dhamma, for, strictly speaking, even the Buddhas who periodically discover it and their Sangha(s) who spread it come and go, are fleeting (as far as their appearance and disappearance, their connection to ‘the world’ is concerned). But the Dhamma is forever the same.
with Metta