I'm new to making my own cold brew at home. When making hot coffee I tend to buy darker roasts or espresso blends because they are less acidic. I assume the same rules apply for cold brew, but I was curious if there is a better/preferred type of bean or roast if the go is a low acidic taste.
There are many variables that affect the final cup (in any type of brewing):
The brewing technique (even with cold brew you can change factors like grind size, steeping in the fridge or at room temperature, brew time, etc.)
The type of roast (as you say lighter versus darker roasts)
The type of bean (different origins and how the bean was processed from a fruit to a green bean)
Type of bean
Let's start with the type of bean. Some origins are more acidic than others. One indicator is the altitude at which the coffee is grown: a higher altitude results in a more acidic bean, which generally leads to more complex fruity flavors. According to a blog by perfectdailygrind:
The main reason that higher elevation coffee is more sought after is the taste. When well-cared for, high elevation coffee will produce the more acidic, aromatic and flavorful cup of coffee that we love, while lower elevation coffee tends to have a lower acidity with little character in the cup.
If you want less acidity, you might want to consider beans cultivated at lower elevation. Two low altitude beans are Brazil's Yellow Bourbon (also popular as a bean for espresso) and Kona coffee from Hawaii (quite expensive and there may be blends with a high proportion of other beans mixed in).
Type of roast
Of course, the darker the roast the more pronounced the bitter flavors will prevail over the acidic flavors. Nevertheless, choosing a dark roast means you will mostly taste the flavors determined by the roast rather than the ones acquired by growing the bean. If you want to have some variety, I would suggest looking for medium roasts so different origins provide different taste.
As indicated in an answer to this question about roast levels and cold brew, cold brew produces a less acidic cup compared to hot brewing methods:
Cold brew produces a more muddled mixture of flavors and relies more on mouthfeel and big flavors like sweetness, savory, earth, chocolate, etc. More delicate precise flavors are lost. It is also important to note that some fruit, vegetal, and herbal flavors are enhanced by the presence of acid, like adding lemon juice and tarragon to a fish filet.
So, if you are trying to avoid acid but you love the fruitiness of say, a medium roasted Kenyan single origin, you could cold brew it and see if you still get the flavors you want.
As for brewing time, I think 18 hours at room temperature is a good starting point. According to this guide by homegrounds.co increasing the time risks getting an overly bitter result while a much shorter extraction can yield a weak cup.
Grind size is one of the factors that you can play with to change the bitterness / acidity tradeoff after you've settled on a particular coffee. The finer you grind, the more you favor bitter flavors over acidity. Remember though that you don't want too grind too fine as that will clog up your filter (regardless of the system you are using) and it might produce a muddy result.
Brew ratios are about the ratio of coffee beans to water. North Star roasters recommend are ration of 1:8 which is one part coffee beans / grinds to 8 parts water (by weight). They describe more concentrated ratios but they mention that affects brew time.
In summary, I think you can get to an enjoyable cup by focusing on just two variables:
Choosing the roasted beans. As stated before, there is less variability if you stick to darker roasts. With medium roasts the taste will differ more based on origin and growing conditions. Beans from Brazil are commonly available and I think they are a good starting point.
Varying the grind size to finetune. Once you've settled on a bag of beans this is probably the factor to focus on. If you keep everything else the same (the same beans, the same brew time, etc.) then you can increase or decrease the bitterness by grinding coarser or finer, respectively.