MichaIng the thing is that this is happening on RPi 4b, not a 3.
However, I found this thread and applied the fix and it’s now working just fine and the backup was done very quickly afterwards whereas previously to that, it would fail and take a while.
Worth notice that I came across this community thread related with the quirks when I started testing the SSD (with the SATA-USB) and noticed that the write speeds were really off when using dd (at kbps). After applying the boot quirks the write speeds were back to ~200 mbps.
Let’s see how it goes from here. 