SCT TRACON Controller reported they did not notice a VFR aircraft climb through the altitude of an aircraft in a holding pattern resulting in an NMAC. The reporter was working combined sectors due to staffing shortages and stated they were too busy to notice the confliction.

Narrative

I was working one sector and I was told to combine up the other sector because we didn't have the staffing to keep it open. To help with the increased traffic management decided no practice approaches and limit VFR services. I was still adjusting to the sectors combined and the increased traffic on 2 frequencies. I had Aircraft X he said climbing to 5500 ft. while Aircraft Y was holding at 4000 ft. I was too busy to keep watching it. I thought he would have climbed above the other Aircraft Y. The frequency was congested; I could barely get a word in and the pilots were not getting a lot of my calls. When I saw the conflict I tried to issue a safety alert but the frequency was partially blocked. Aircraft X had leveled off at 4;000 feet and flew straight through Aircraft Y at the same altitude. Another example of why short staffing is a safety issue. This could have easily been an aircraft accident report. We got a free lesson here and the FAA needs to figure out the staffing quick. Our mental health is suffering with the extra stress and close calls like this. Plus mandatory 6 day work weeks; it's too much and not enough rest or time to recover.