The latest Librespot compile seems more stable. I now use my Spotify Premium username/password and disable discovery. Early days still, but it’s working 100% reliable (including reboots) over the last few days. Here’s my current .service definition:
[Unit]
Description=Librespot
Requires=network-online.target
After=network-online.target
[Service]
User=root
Restart=always
RestartSec=10
ExecStart=/root/librespot --name SpotiPi --username renebouwmeester --password ******** --disable-discovery --cache /tmp --bitrate 320 --device hw:0 > /dev/null 2>&1
[Install]
WantedBy=multi-user.target