Hey, that Orbea Onix is a sweet ride!
If it's too large, why can't you just downsize it? Why trade it for the Trek?
Go in today and discuss the fit issues.
Otherwise, that's a nice bike... don't think you bought a bad bike. You didn't. Those are great bikes. Just dial in your fit with the shop.
It will be fine.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"