1) Is somebody running bitcoind with i2pd? Is there the need to configure some i2pd conf files?
I tried i2pd today, and I also notice it keeps restarting. Seems there is something wrong with ip2d v2.39 and Bitcoin Core.
The last log before it got SEGV was:
15:52:25@973/error - Garlic: Failed to decrypt message
2) Do I need to open ports on the router of my home network? My intention would be to have only outgoing bitcoind traffic. Not sure why I would need open ports for incoming one - but maybe I don't understand how i2p works?!
Nope.
I2P will work fine when firewalled, there is no reason for concern. When firewalled, the router uses "introducers" to relay inbound connections.
You might want to try to update it to the latest version. Or just use the official implementation, I2P Router, I do use it, and so far there haven't been any problems.
The installation could be done by following:
https://geti2p.net/en/download/debian#debian. Then enable the SAM on the configuration page(
http://127.0.0.1:7657/configclients). With the corrected bitcoin config, you should be good to go.