Not suggesting the first mix of an untried recipe should be 300ml but I would argue that should a beginner find a mix that they really like and have tested after steeping the fact that they have not got a backlog of mixes would be a good reason to mix a large batch month steep time or not.
As far as mistakes well my thoughts are opposed slightly yours on that as well in that an error in a small mix is much more noticeable than that same error in a large mix, unless the error was grabbing an incorrect concentrate which I will admit to doing on a 1 lt mix. I was lucky it gave me another recipe as it turned out really nice . I do agree there would be less wastage if you had to tip a small mix.
I guess what I am saying really is that I don’t think that advice to not mix a large tested batch is valid for a beginner or anyone else. This obviously is just my opinion.