This is about section 3.3.1, steps 1 to 5, correct?
What have I done wrong? What can I do now?
From your description you seem to have omitted step 4: Does the message appear?
If not, I assume step 1 or 2 are not correct. I notice that the boot image has a different name (appears to be the regular boot image), but I know nothing about how to boot a Volla / Gigaset phone reflashed to SailfishOS into recovery mode, because I do not own one. Maybe @poetaster or @piggz can provide an advice.
If only step 5 fails, take a look at the link there.
[…] mounted partitions can only be enlarged, not reduced.
This is correct, hence one has to boot into recovery mode to enlarge them, because then they are not mounted.