I work on the premise
the lower, the better, even though my body sometimes disagrees.
I've been low carbing for years, and have to say that for me, death on toast only happens now with carbs.
However, my body likes the 5-7 mol/l range. It has never liked or been comfortable in the 4s, and nothing that I have ever tried has made any difference to this. I have long ago tried to stop forcing my bg below 5.1ish, because I feel pretty grim, and I never get used to it.
If I stray higher than my comfort zone, I start to feel bad at 8+ (and then feel cruddy for a day or two)
If I stray lower, (say 4.5 or below) my body starts to do the fluttery panicked hypo-incoming thing - but then I have reactive hypoglycaemia so I don't really know what it feels like for a normal T2 at those levels. I have seen some people write that they don't feel anything til they hit the low 3s. I envy them.
I suppose it really boils down to habituation. The body gets used to its usual running level. Stray out of that, and you will start to feel 'wrong'. The more you get used to something, the weirder it feels to stray outside it.
But it is well worth nudging your comfort zone downwards, if possible. As I said,
the lower the better (within reason! lol) for your long term health.
I am 49 and feel better than I have for maybe 25 years. It was going very low carb that did it. Kind of like getting a body transplant, to be honest, but there are plenty of people on here who are much older and who run at better blood glucose levels than me - part of it is the hand you are dealt, and part of it is being willing to push through the temporary adjustments that changing ways of eating/lifestyle and exercise require. I remember when I started walking the dogs I had backache for maybe 6 months, then one day it just... wasn't there. My back is now hugely improved on how it was pre-dog, but some days during that 6 months I was crying in pain.