My recommendation would be to copy your entire home directory with rsync -a onto another (external) drive, as you anyway don't want to modify your partitions without having a backup.
Then boot into a live distribution and open a partition editor, delete the home partition (the data on it will be lost), expand the root partition (/) onto the entire disk. Finally copy the backup back into the home folder using rsync -a
If sudo apt -f install doesn't work properly, you can create an apt-cache folder on, e.g. your home partition, assuming this is the one with sufficient amounts of free storage.
I just installed the Flatpak version of Inkscape (the first GTK application, that came into my mind, where it makes sense to have the latest version installed) and the Breeze dark theme works for me.
Concerning the compatibility, there shouldn't be a difference between LMDE and Debian. The choice of the Desktop environment depends on your personal favour. As Mac user, you probably tend towards Gnome which can be installed during the setup of Debian, not LMDE. However, as I don't know about the performance of the 2013 iMac, maybe Cinnamon or Mate are a better choice.
A spokesman from the extreme right party AfD once got cited with the words: "Je schlechter es Deutschland geht, desto besser für die AfD." (The worse Germany is doing, the better it is for the AfD.)
I think this highlights the underlying reasons why the AfD wants Germany to leave EU.
Do you really have to reinstall from scratch or is it sufficient to update the sources.list to the new Debian release and perform dist-upgrade like for Debian?
My recommendation would be to copy your entire home directory with rsync -a onto another (external) drive, as you anyway don't want to modify your partitions without having a backup. Then boot into a live distribution and open a partition editor, delete the home partition (the data on it will be lost), expand the root partition (/) onto the entire disk. Finally copy the backup back into the home folder using rsync -a