Short-keys can be assigned to the following commands:
amixer -q -D pulse sset Master toggle
Volume up by 5%:
amixer -D pulse sset Master 5%+
Volume down by 5%:
amixer -D pulse sset Master 5%-
For a panel/tray button, install
In Debian, Ubuntu:
sudo apt install qasmixer.
To have it in system tray, go to File-Settings-System tray and, in case the tray icon doesn't appear, un-check then check again the "Show tray icon" option. Be sure to check also Mini mixer device - Current so that the tray icon actions like mouse scroll act upon the desired device.
Normally it will control volume of all kind of speakers. In case it stops controlling the bluetooth speaker(s), check File-Settings-Startup and View-Show device selection-Mixer device.
Make it start with the system by adding it to startup application list with an argument for
start minimized in tray:
Sources here and here.