You could script something like this:
First get the list of sources:
gsettings get org.gnome.desktop.input-sources sources
e.g.
gsettings get org.gnome.desktop.input-sources sources
[('xkb', 'gb'), ('xkb', 'us'), ('xkb', 'th')]
Then in your script change around the order - sleep and then reset back again. That seems to work for me e.g.
gsettings set org.gnome.desktop.input-sources sources "[('xkb', 'th'), ('xkb', 'us'), ('xkb', 'gb')]
sleep 3
gsettings set org.gnome.desktop.input-sources sources "[('xkb', 'gb'), ('xkb', 'us'), ('xkb', 'th')]
This as an autostart script should work
SOURCES=`gsettings get org.gnome.desktop.input-sources sources`
gsettings set org.gnome.desktop.input-sources sources "[('xkb', 'de')]"
gsettings set org.gnome.desktop.input-sources sources "$SOURCES"
The second line doesn’ matter since it is reset by the third line.