BE-33 pilot reported due to radio congestion; pilot initiated a descent upon entering moderate to severe turbulence and icing resulted in clearance deviation and momentary loss of control.

Narrative

IFR departure from ZZZ. Clearance was after takeoff; heading 180; climb to 3;000 ft; contact ZZZ Approach on XXX.XX. After takeoff; began a climbing left turn to 180 and checked in with ZZZ Approach. Cleared to climb to 5;000 ft; then assigned heading 120. Passing 4;400 ft; encountered continuous moderate turbulence and became concerned about possible precipitation and icing and turned on my Pitot Heat. At 4;700 ft; turbulence increased to moderate-to-borderline-severe; aircraft experienced abrupt attitude changes and difficulty maintaining heading. Light rime ice began forming on the wings. Radio frequency was congested; and due to the intensity of turbulence I initiated a descent without prior ATC coordination. As soon as able; I advised ZZZ Approach that I was descending to 4;000 ft due to 'moderate turbulence and icing.' My voice inflection reflected the aircraft's abrupt and turbulent motion. Controller responded that I could not change altitude without approval and there was traffic below. He issued heading 140 and descent to 4;000 ft. Turbulence lessened but I was momentarily shaken. After turning to 140; I inadvertently failed to continue descent and began climbing back toward 5;000 ft. Controller queried my altitude; I apologized and resumed descent. A new controller took over; instructed climb to 5;000 ft and direct ZZZ [VOR] when able. I complied and reported that the earlier turbulence was moderate to borderline severe. A few minutes later; controller instructed heading 180 direct ZZZ1 [airport] when able and asked if conditions had improved. I advised that weather was now smooth and VMC. Controller confirmed my prior report; and I clarified that passing 4;400 ft I had encountered moderate to borderline severe turbulence with light rime icing. Controller asked if I needed any assistance and if there were any injuries onboard; I replied none. Remainder of flight was uneventful.