Customize pop-up indicator position and size for power and volume

When changing the volume (by pressing the keyboard keys) this icon pop ups, right in the middle of the screen:

I would like to change both the position and the icon, ideally to a more discreet location on the screen. I've seen other questions asking how to change the icon completely, but haven't found how to change the position and size.

Edit: I suppose whatever process works for the volume indicator also works for the brightness indicator. (Not sure if "indicator" is the right word for these icons.)

Better OSD is what you're looking for. It works very well on Ubuntu 18.04. It is a Gnome extension, that is available via Ubuntu Software too.

After installing it via Ubuntu software, you can access to its settings via Gnome tweaks>Extensions (if you don't have Gnome Tweaks, you can install it from Ubuntu Software too).
Here are some screenshots of what I got after playing around with the extension settings:

You can even resize the volume indicator

PS: If it doesn't work for you directly after installing it, you may need to restart the gnome shell environment ("logout and login again", or Alt+F2 then R then Enter)