Hi Jack, first of all thanks for this valuable script!
I wanted to use the script but it seems speedtest did some changes.
if you folow the docu there is a small change … speedtest is now known as speedtest-cli, in the repo there is no longer speedtest so apt-get install speedtest will fail, but apt-get install speedtest-cli does install speedtest.
As you described there is a cli version but it seems this is the only one availible now 
pi@nms:~ $ speedtest-cli --version
speedtest-cli 2.1.3
Python 3.9.2 (default, Mar 12 2021, 04:06:34) [GCC 10.2.1 20210110]
After some testing there is a way arround:
Alternatieve for pi4 wget https://install.speedtest.net/app/cli/ookla-speedtest-1.2.0-linux-armel.tgz and unpack that one does work and is the speedtest first time you need to agree with the licence
version is:
Speedtest by Ookla 1.2.0.84 (ea6b6773cf) Linux/arm-linux-musleabi 5.15.76-v7+ arm/armv7l
And this one suits i copyed it in the /usr/bin folder and should be working … keep you posted