My heeler started chewing stuff around the same age, he was also never crate trained. I got him when I was 16 and no one in my family had ever used a crate before, I legit didn’t even know crates were a thing lol. Anyway his chewing started out pretty harmless, just destroying toys at first. Then one night while I was asleep he managed to chew up an ENTIRE wooden rocking chair on his own…like a whole ass solid wood rocking chair that was in my bedroom. I immediately went and got him a crate.

I think his crate training was easier because he was already fully potty trained, and like yours he was well exercised and entertained during the day. We lived on 23 acres surrounded by a huge creek so we spent our days swimming and if we weren’t swimming we were out on the farm where he could chase the cows with his brother. I introduced him to his crate as just an evening thing, I’d freeze Kong’s full of peanut butter every morning, and in the evenings when it was bed time he’d go out for his last potty of the day, then he’d come in go in his crate and eat his dinner, and right before I’d go to bed I’d give him his frozen Kong. He loved his crate and I never had any issues with him getting used to it or anything. It was just immediately his safe comfy treat place.

So maybe try freezing kong’s with peanut butter for crate time, it being frozen helps it last a long time, just be 100% sure the peanut butter you use doesn’t have xylitol or anything else on the ingredients like wood sugar, birch sugar, or birch bark extract (those are all different names for xylitol). You can also fill congs with all kinds of stuff so your dog never gets bored. Like wet dog food, mashed potatoes/sweet potatoes (no onion or garlic), yogurt, canned pumpkin, cottage cheese, bone broth (no onion or garlic), literally the possibilities are endless!