Beechcraft Bonanza pilot reported landing safely at departure airport after experiencing a significant power loss shortly after takeoff.

Narrative

Me and a CFI friend of mine were picking up a J35 bonanza with the intention of flying it down to my home base. The aircraft had no issues when I had flown with the previous owner and had no issues in the run-up (Mag check lost approximately 100rpm on both sides; prop cycled correctly; and all engine instruments were reading in the green). We were cleared and started our takeoff roll off of runway XXR out of ZZZ. There were no issues on the roll-out and we lifted off at 75mph. When we past the end of the runway I retracted the gear. We climbed out at 500 fpm for approximately 2 minutes and were switched to approach frequency. Around 1000 feet AGL we experienced an RPM loss of approximately 750rpm. I immediately turned back toward the airport; simultaneously I made sure the mixture prop and throttle were all fully forward; and [requested assistance]. I was initially asked to enter a left downwind for XXR; by ATC; but then realized that the engine wasn't supplying sufficient power to hold altitude. I pitched for best glide and told ATC that I was not able to land XXR and would be landing opposite direction on XYL. I did not pull the power; flaps; or gear until I was over the numbers and had the runway assured. I landed on XYL and pulled off the runway to [an] FBO. While taxiing I felt the engine popping and backfiring; which became worse below 1500RPM. I have yet to figure out what caused the engine roughness; but it is being seen by an IA in ZZZ. I was new to the aircraft and if I had more time in the aircraft I may have seen warning signs prior to an incident occurring. There are always ways to improve on how you react; in hindsight I would not have originally accepted a turn toward a left downwind for ZZR. If I had lost my engine completely at that point I may have not been able to make it back to the airport with the altitude that I was working with. There was an unmarked runway-like-area next to the airport that may have sufficed but I'm not sure how long it was nor how wide it was. Moreover I would have done more flights the days prior in the aircraft. This would have made me more familiar with the aircraft and possible warning signs of the issue may have come up. I also was new to the area around the airport and flying more flights in the area would have given me better situational awareness in the moment.