I cheated. My MacBook project was doing well but I had to go too far. My £200 experiment in updating an 11 year old Mac ended with me using a version of MacOS Catalina. Until a week ago this was the latest one. You could use a website run by a developer to "hack" a copy that would work on my ancient Mac.
However, it was unsupported. The IT lesson is always that "unsupported" is not the same as "doesn't work". Supported means, in this context, that Apple will update the OS with a variety of ongoing security updates and maybe even a feature update. This latter possibility being unlikely.
The supported OS was High Sierra. This is the 2017 MacOs. To be fair none of this is important to me specifically. Getting a really cheap 11 year Mac working was the exercise, not what OS was installed. The fact is that I got quite a few browser crashes and, from what I could see, a considerable performance hit from trying to get it all working.
I am off work and locked down. So today I re-instated High Sierra, version 10.13 of the Mac family.
In the paraphrased words of Clint Eastwood; "A man has to know his limitations". Today, my limitation was High Sierra.