Is there any way to add a launcher to the dock (optimally Slingshot, but any launcher would be great)? It seems odd to me as a matter of design and efficiency to have to go to two different locations to launch applications. A single location in the dock would be great -- either as an option or as the default.
You can use Cardapio - I think it's in the repositories, so type
sudo apt-get install Cardapio.
Then find Cardapio in slingshot and add it to Plank. I know it's not slingshot, but it works just as well, although you may need to use Menu Editor and hide some of the unnecessary entries.
Personally I use DockBarX with Cardapio, but before that I used Plank so I can tell you it works.
You can try Synapse
sudo add-apt-repository ppa:synapse-core/ppa
sudo apt update
sudo apt install synapse
Also if you want, you can remove slingshot entirely:
sudo apt remove slingshot-launcher
You could try doing this via a sh script, then create a .desktop file as a launcher to add to plank. I've never done it myself, but that's where I would start
In scratch create a new file:
wingpanel --toggle-indicator app-launcher
save the file as launcher.sh or whatever.
chmod +x launcher.sh
...to make the sh executable.
Then create your desktop file and save it to ~/.loca/share/applications or for all users /usr/share/applications. Make sure the file has a .desktop extension, e.g, launcher.desktop.
Comment=Wingpanel Plank Launcher
I've never done this myself, but it's a place to start. And more fun than installing an app to do it for you!