Alright,

Sometimes the solution is pretty obvious but you fail to spot it.
@tekka thanks for pointing me to the correct direction, as the problem was in the used frequency.

Even though I initially tried with 434 MHz, it turns out that when i finally did it right, i had already switched to the default 868.
After adding
#define MY_RFM95_FREQUENCY RFM95_434MHZ
in the sketches initially, i established communication.

After that i also tried to configure again the rasperry (this time knowing that the node is working) and Hooray! it worked :)

Thank you all for your support and @titvs @mfalkvidd as well.
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