Is this the right way to use dd for a full backup?

Can I trust the backup I created in this way?

Yes, of course! This is the most accurate backup method.

Data integrity can be verified by comparing the outputs of running md5sum(1) on both the /dev/block/mmcblk0 volume on your device and the mmcblk0.raw disk image on your computer.

Additionally, you can use parted(8) to list all the partitions on the backup file:

parted /path/backup/mmcblk0.raw print

There you will find your system and userdata partitions, among many others containing bootloaders, firmware and recovery images.

There was a delay of a few seconds between launching the dd command on the device and the nc on the host computer. Doesn't this mean that I've lost some of the output of dd?
Shouldn't these be run concurrently or, at least, shouldn't the nc command be started before the dd one?

Don't worry! When you run nc -l, it will listen for an inbound connection, blocking the pipe and keeping dd from sending data until you establish the connection from your computer.

Note: you don't need to run adb forward tcp:5555 tcp:5555 twice.

Warning

Most of the modern devices encrypt all your data. Make sure that you can decrypt the backup file and access your data before erasing the phone.