During isolation I bought my first mechanical keyboard (a cheap Qisan Magicforce 68, that I wholeheartly recommend). I quickly learned to use it and still love it at home.
A few months ago I bought a kinesis advantage for my office. I'm fully used to it (although it took me a while to stop hitting wrong keys).
When I bought both keyboards, I already knew how to touch type and went back to typeracer, keybr and monkeytype. My speed did not increase that much and I still get my fastest speeds on my macbook pro integrated keyboard.
The main reason why I use an external keyboard is to solve my back problems, by using a stand or an external screen.
In particular, I get my slowest typing speeds on my kinesis advantage. It is NOT built for speed, it is built for comfort. It allowed me to improve my posture considerably so it's still worth it.
PS: in my experience, using a laptop stand / external monitor helps with the lower back problems (weirdly). I solved my upper back problems by doing more exercise, and it's as easy as having some elastic bands near my desk and trying to stretch them horizontally behind my neck a few minutes a day (when I'm reading a long email or pdf or thinking or need a break).
PS2: a must-have on macOS is karabiner. I use it to have nicer keybindings for french accents (modifier keys like `) but also to configure function keys on my kinesis advantage and put navigation shortcuts on my mouse (logitech G502).