Your BMW will tow an average 4 birth no problem. Diesels have alot of torque and this helps when towing. The BMW engines are strong and economical, keep what you have and enjoy it
The weight and performance figures between the 3 cars you mention are negligible in reality and if you're happy with your BMW it's probably not worth changing it for either of the others.
I tow a 1250kg 4 berth caravan with a Passat 2.0 estate and the power is fine. I went for the Passat over the Tiguan as it has more space inside, better fuel economy and lower company car tax, and I decided that mattered more than 4WD. But it's really down to personal preference, I don't think you can go wrong either way.