Digging and chewing are self rewarding behaviours that come out mostly from boredom. They can be prevented by LOTS of training, interactive toys and long walks and are very hard to stop when it starts to happen.
Keep in mind that when you take a rescue dog into your household, they might not show their true personality for several weeks as it takes time for them to adjust to new household. The dog who seems calm might turn itno wirlwind then it finally settles.
It might not be the advise you are asking for but I think, that being first time dog owner you might be getting yourself into something that would be difficult to deal with latter.
I see things that raise red flags for me that go beyond your initial question.
You never owned dogs before.
You are looking at the mix of two very active breeds.
You already have SN toddler who I assume takes up a lot of your time, adding high needs dog like that would be like adding another child to your household. Can you budget at least two hours a day for one on one interaction with your dog? Look how many threads on this board that deal with toddler/baby/dog tension and issues, they all have common advise - Train,train,train, exercise! And in most cases those issues could have been prevented by following that advise in the first place.
I am not suggesting that you should not own a dog at all. But I think that you need to ask yourself what are your reasons for wanting a dog in the first place? Maybe less active breed like a greyhound (known as 40mph couch potato) would work better for you than active mix breed.
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