I am not sure whether this is easier then yours, but that is how I do it.
- I make an iso image from the sd with Flash Drive Image Creator (It is time and space consuming, but I want to have backups of the sd before and after editing anyway).
- Then I use PassMark OSFMount to mount the partitions (from the iso image) and do the changes.
(In order to access the Linux partitions [also with PassMark OSFMount] I use Ext2Fsd.
- I write the image back to the sd with Flash Drive Image Writer.
The location of Flash Drive Image Creator and Writer has changed several times in the past, currently (May 2018) you can find them here:
https://www.vconsole.com/download