I got the 42099 for Christmas as well.. Build the standard model on boxing day.. Not had any Lego for years.. Today I built Rocky!
BuWizz app works with the control+. If you have a go make sure you bind both drive motors! On the standard model it appears to allow the steering to go beyond its limits.
I had the same problem with the steering being off, I just pulled apart the front end and adjusted the center point, it seems better now.
Back to Rocky.. Big shout out to @Didumos69 what a great design, it performs so much better than the standard model. Found the build trickier than the standard model which was a good challenge.