12-hour fermentation makes dough wetter After bulk fermentation of my dough for 12 hours, I find that the dough is too wet or sticky. Should I bulk ferment for a shorter period of time, and retard it in the refrigerator?

I'm currently baking cinnamon bread with nuts. The recipe is 600g bread flour, 200g warm water, 200g warm milk, 150 grams chopped unroasted walnut or pecan nuts. 2 tsp active dry yeast, 10g salt, 30g sugar.

After combining the above ingredients, the dough looks and feels perfect and taut. I leave it to bulk-ferment overnight (not "overday") in room temps of 29-30 C.

Here's the problem:
After bulk fermentation, the dough is slightly wetter and thus too sticky. I could knead it for 15 minutes, and still wouldn't come-together. During the final proofing, it would spread towards the edges of my loaf pan, but wouldn't rise above the rim. In spite of scoring it 4 times and spraying it with water every 10 minutes inside the oven, the bottom still cooks much later and slightly dislodges the crust that set much earlier.
You are overproofing your dough. 2 teaspoons dry yeast, 600g flour and a warm rest of twelve hours sounds like a recipe for disaster to me - not to mention that half of the liquid is milk, which would warrant a closer look into food safety topics as well.

Also, any extended kneading (if used) is usually prior to bulk fermentation, possibly with an autolyse step before adding yeast, but not after a bulk rise. You might choose to "punch down" your dough depending on your desired crumb structure, but kneading renders the previous bulk fermentation moot.

Taking your recipe and environement into account, I would expect your first rise (= double volume) to be complete in under an hour. It seems that due to enzymatic and yeast activity your dough lost its internal structure and the gluten network started to break down. The fact that your dough tends to flow outward, not rise up is another indicator. You can slash and steam as much as you like, once the gluten network broke and the yeast is exhausted, an overprooved dough will not yield good results.

For cold fermentation in the refrigerator the rule of thumb is 1-2% fresh (cake) yeast / 0.33 - 0.66% dry yeast, based on flour weight. For your 600g, that means between 2 and 4 g (or between a generous 1/2 to 1 teaspoon). Use cold ingredients and rest for 8-24 hours. The shorter you want to rest, the higher the yeast ratio, obviously.