RV6 pilot reported a near miss with a C172 aircraft while in descent.

Narrative

I was on flight following with ZZZ TRACON to ZZZ. Ceiling was unlimited; visibility was decent with slight haze and glare from the sun. I began a VFR descent from my cruise altitude of 5;500 MSL. ZZZ [TRACON] frequency was very busy; so I did not inform the controller of my intentions until I had already descended a few hundred feet. Shortly before notifying the controller; I noticed a potential traffic conflict on the cockpit ADS-B traffic display; and I programmed the autopilot to turn right to avoid the indicated track of the traffic. Upon notifying the controller; he responded with an urgent tone of voice 'do NOT descend'. Simultaneously; I noticed the separation between my airplane and the traffic on the traffic display had become much less. I reacted by disengaging the autopilot; pitching up and starting a climb; and steepening the right turn. As I began the evasive actions; the traffic briefly appeared in the lower-left portion of my windshield before disappearing underneath my left wing. The vertical separation was likely less than 200 feet; and the horizontal separation may have been even less (the airplane appeared to be almost directly below me). Upon verifying passage of traffic behind the airplane; I resumed normal navigation.I believe that during the entirety of my descent until the evasive action; the conflicting traffic was in a blind spot beneath the windshield; blocked from my view by the engine compartment. I was flying a direct course from ZZZZZ to ZZZ; with a magnetic heading of approximately 095. The traffic was flying a northwesterly heading; approximately 295 magnetic. The traffic did not appear to change course or altitude; and did not appear to be aware of my route. The traffic was a high-wing airplane (appeared to be a Cessna 172). I do not know whether I was blocked from their vision by the high-wing blind spot.I believe the contributing factors to this incident were the possible impairments to visibility caused by haze and sun glare; delay of communication caused by radio congestion; my failure to properly anticipate the traffic conflict; and my failure to disengage the autopilot and take evasive actions more promptly.