To cut a long story short…
The editor provided by Github is very basic and not exactly feature rich. I decided I needed something better and browser-based so I could work from anywhere.
I tried StackEdit which is a decent editor but connecting it to Github wouldn’t work. I looked at JekyllPad which didn’t quite fit my needs. Then came Dillinger and a few others.
On the verge of giving up, I searched for “web-based markdown editor for github”. And now, I feel a bit silly because the number one item in Google’s AI Overview suggested I try " GitHub’s Built-in Editor (GitHub.dev)".
Yep, the very thing I was looking for was on GitHub all along. And, it turns out to be a web-based implementation of Visual Studio Code - which is what I already use on the desktop.
Duh!