Ivy

How cold is the water in practice? I have been using tap water without thinking about it.

Second the over-fermentation, and add one thing: the float test tells you a starter has gas, not that it is at peak. Tripling in four hours means you should be using it well before four hours, at or just after peak. A starter that has already begun to fall is more acidic and degrades gluten faster in the dough.

Pour the grey liquid off rather than stirring it in if the smell bothers you, though stirring it back is harmless. It is called hooch and it is basically a hunger flag with a name.

A band at the bottom specifically is usually underbaked base rather than a fermentation fault, and 96C internal is on the low side for a sourdough loaf. I bake until 99 to 100C in the centre, which usually means another 8 to 12 minutes uncovered. The base is the hardest part to cook because it is against a hot mass of iron for the first half and then insulated by the loaf's own steam. Try lifting the loaf onto the bare rack for the last ten minutes and see if the band vanishes.

If the fridge is genuinely full, an unheated garage or a cool porch in winter does the same job. I retard at 8 to 10C outdoors from November to March and it is fine, just faster.

Cold dough holds its shape better under the blade and gets better spring. Warming it up mostly gets you a flatter loaf and a slack score.