If the ECA was too tangy, you can just use less, if you are happy with the results of your sausage else wise.
The amount we typically recommend (4 oz per 25 lb meat) is at a level to try and lower the pH enough to get as close to reaching more of a shelf stable product as we can. But, it doesn’t have to be used at that high of a usage level.
I can’t recommend to use buttermilk, because I’ve never used it. The concept kind of makes sense though. You are adding something with a lower pH than the meat, to get the meat pH just a bit lower and add some tangy flavor. The biggest problem I see though with using something like buttermilk and immediately lowering the pH is this… A lower pH will reduce water holding capacity, and can make a sausage dry and crumbly after cooking. This is one reason why encapsulated citric acid is coated, and that coating doesn’t melt until 135 F +. If you add buttermilk and lower that pH immediately, I am not sure that the results will be quite the same, and you could end up with a dryer or crumbly sausage. I’ve never used buttermilk though, so I can’t say for sure, but that’s my thoughts anyways.