Bread – replace sugar with cane syrup in sourdough recipe

You can easily replace the liquid in most bread recipes with beer. This can have a very pronounced effect on your final dough as there is a lot more chemical and biological fun happening in beer than there is in water. In my experience, the dough with beer will usually rise faster than a similar dough with water. Generally, the flavor difference won't be that pronounced (usually a much more "bready" flavor, unless you use a beer that otherwise has a strong flavor, such as an IPA).

The acidity of the beer won't actually have that much of an effect on your final dough as the ph will be a weighted average of all of your ingredients.

Adding milk works because milk is mostly water. It might even work better, as it also contains sugar (lactose) which will be eaten by your starter's yeasts and bacteria.

But it also contains other stuff, such as fats. And fats, after some time, get rancid. You probably won't want that taste in bread or pancakes. I would stick to water (which quite probably you'll find cheaper than milk).

Don't worry about rotten milk: your starter is already rotten. And it's a hostile environment for most microbes. It's very very improbable new microbes would survive there. But if they do, they'll be quite similar to the already existing ones.