Can you enter recovery ?

https://together.jolla.com/question/...-in-boot-loop/

If so, do a factory reset from there, and update device afterwardd.
A full dd of mmcblk0p28 is ~16Gb a bit much for uploading somewhere.

for a system flashable image. It should become far smaller as a dd is not needed, but should be created from filesystem. Not including blank - not used space