But my question is, how much testing should I do?
I'm in America, where your HbA1c of 5.6% would be "normal," not pre-diabetic. Mind you, eight years ago I was about the same as you (5.5%) and here I am now, squarely diabetic as of February of this year, having been given no warning eight years ago and being blissfully unaware of "HbA1c" at the time even though I had been tested for it.
I cannot answer your question about testing but I can recount my experience. When diagnosed with T2D nine months ago I decided to eschew testing and just adopt a low-carb diet, as recommended by my doctor. Six months later my A1C was lower than yours, at 4.9% (30). Now this is not to say you shouldn't test, but I do think people should put the "no testing" option on their list.
It is partly a personality thing. I get anxious about things quite easily. Your post is an example of the kind of things that could easily gallop through my mind if I were testing daily. I'd rather not, unless my numbers eventually worsen without apparent cause.
The advantage of testing is that it makes it much easier to fine-tune your food/drink intake for your particular body, since we are all different. But for me it was easier just to adopt a pretty extreme diet and stick to it. I am fairly sure that my approach would work for a lot of people who are now testing, but that does not mean it would be the right approach for them.
Well done for losing so much weight, and for working to take things under control.