Pilot reported a NMAC while departing IFR as a result of ATC not separating traffic.

Narrative

When departing ZZZ Runway XX IFR en route to ZZZ1; I was given a takeoff clearance with instructions to maintain 3000' MSL and the runway heading. After being handed off to departure; I had to make three transmissions to ZZZ Departure (XXX.X) to establish two-way radio contact; likely due to distance from the radio antenna and/or frequency congestion. The controller informed me that they had made radar contact and that there was traffic at my 2 O'Clock at 2800' who had me in sight; before talking to other aircraft. I saw the converging traffic within a few seconds; and seeing that we appeared to be on a collision course; I immediately stopped my climb at roughly 2400'. The frequency was congested; but I was able to tell the controller that I had leveled off to avoid a conflict; at which point they issued me an amended clearance to maintain 2400'. After the traffic passed directly above me; I was able to resume the climb and the flight continued without incident. I believe that without violating the clearance and quickly leveling off; there would have been a very close call that could have resulted in a collision. While being in VMC means it is my responsibility to see and avoid; the IFR clearance from ATC had me on potential collision course with another aircraft; who was also in ATC contact (I believe VFR). The other pilot; who said he had me in sight; made no apparent effort to climb or change course. Typically; when departing ZZZ IFR; I am assigned a departure frequency of XXX.XY; which is usually not very congested and primarily handles ZZZ2 traffic from my experience. On this occasion; XXX.X was handling ZZZ and ZZZ2 in addition to ZZZ3 departures; which made it very busy and was likely a factor that led to this oversight from the controller. Also; the ZZZ tower controller told me to hold short of the runway with traffic in the pattern; but then issued me a takeoff clearance with no delay and heading/altitude instructions. It seems like there may have been a rush on the part of the tower/departure controller to get the IFR release and move on to the next aircraft. While this conflict was realized before it became a serious near miss; the fact that the Departure controller issued an IFR aircraft a traffic advisory instead of trying to resolve two covering targets is worrisome.