I could have written your op 5 years ago.
I had a border collie from Ireland, from Dog's Trust. Basically everything that the behaviourist from the Dog Trust told us was wrong and made a hard job even harden.
I had the stairgate that he jumped, destroyed a settee, destroyed a carpet, chewed tables, emptied kitchen cupboards and many other things.
Had clearly not been in a house and first time we brought him home took us 30 mins to get through the door.
Was advised to attend dog training which was a big fail, basically everything we was told to do was too full on. We should have let our dog acclimatise to us first and circumstances and we probably traumatised him.
For twelve months I persevered although I spent so many nights crying on the phone.
Somebody then recommended a behaviourist in Walsall to me and they were a miracle. Literally had one visit and it was like a light came on. We had tried everything and most kept saying we had to be in control, etc and tried pack training.
This was so wrong, one thing was jumping up so had loads of cheese cubes (high reward) and in the split second he had 4 paws on the ground I gave him a reward, this worked for other things to. It was relentless watching him to catch the exact moment to reward with the cheese but positive reinforcement worked.
He also had seperation anxiety so I moved the room round so he could sit on the settee in the window watching for us. Also had a kong just for when we went out and dipped it in cream cheese, he then realised this was his "reward" for us not being there.
It was incredibly hard but I now have the best dog ever and I am so glad we stuck with him. He is so loyal
Good luck