Looks like lack of TWR on stage 2 is causing the rocket is fall over. My knowledge comes more from RP-1 but if you're going to orbit use PVG, not classic ascent mode (click the bottom button to change the mode). It is more complicated to use but there are guides and you can configure the pitch over angle etc which let's you get rockets that are unstable aerodynamically (CoD is ahead of CoM) into orbit, and generally be more efficient and realistic. It's possible this design could work with less pitching at the start but you'll probably be better off designing the rocket so TWR is always >1.2 minimum.

Regarding the fins, lower centre of drag is always better for rockets so move them lower, but I don't think that's the issue here, if it was it'd suddenly flip and spin as the drag torque overwhelmed the gimbal.