Is it possible to switch between apps like in Unity by Super+1..9 shortcuts? This is the fastest method to switch between them and it's not a problem to remember 5 top used applications that are placed on first positions in launcher.
You have a couple of options.
You could use workspaces. IE keep Firefox in workspace 1, Geary-Mail in workspace 2, etc. Then you could set super+1 to take you to workspace 1, super+2 to workspace 2, and so on. To set shortcuts go to "system settings > keyboard > shortcuts > workspaces"
If you do not want to use workspaces, you could use wmctrl. If you go to "system settings > keyboard > shortcuts > custom" you can set a hotkey to run any terminal command. Wmctrl is able to list, raise, and lower windows from the terminal. Here is an article on how to use wmctrl (complete with a dissertation on why it is better than using workspaces) to do what you are trying to do https://vickychijwani.me/blazing-fast-application-switching-in-linux/