Crosstek has around a 500kg payload.
A receiver and drawbar/ball is probably 25kg.
4x 75kg people: 300 kg
1 large dog 
or cargo 40kg
That gives 135kgs to work with. So max 297lbs tongue weight.
Something has to give. Either passengers, cargo or you are not pulling 3500 lbs with a 10% tongue weight. I didn't even include the standard roof box.
Many vehicles face this dilemma though.
EVERY vehicle with a tow rating has this dilemma. pickup trucks included. Max tow rating on any half ton truck is with a driver, a single passenger, and zero cargo in the vehicle.
And, if you're going camping with 4 people and a large dog, with a roofbox, pulling a trailer, with a crosstrek, you bought the wrong vehicle.
Look at the intention here: This is ideally for a couple, either young or old, who want to do a bit of adventuring or touring. That 3500lb tow rating isn't for pulling a single axle travel trailer that weighs 3499lbs dry, it's for pulling the 1200lb pop-up, or 1000lb teardrop trailer without any issues at all.
I call out half ton drivers all the time. No way anybody with a half ton should be maxing out their tow rating on the regular when pulling a travel trailer. A 34' 12000lb travel trailer is NOT half ton towable, even though an F150 may have a 13 100lb tow rating.
