Tower Local Controller reported an aircraft on final approach initiated a go around due to traffic crossing the runway.

Narrative

I just got in for the midshift. I combined Local 1 and 2 as we normally do. We had gone from configuration 4 to configuration 1 the calm wind runway. The arrival rate quickly escalated as we were getting arrivals to both [runway] XXL and [runway] XYR. Due to the amount of arrivals to XYR I couldn't depart XXR so I was bringing a GA across XYR to XYL to set up to depart. With my focus on this side of the airport I did not notice the compression of Aircraft X and the Aircraft Y on XXL. When I scanned back to XXL I issued exiting and crossing instructions to Aircraft Y at Taxiway 1. I visually saw Aircraft Y exit the runway and then crossed another aircraft holding short of XXL. I then scanned the final and saw Aircraft X executing a missed approach about 500 feet down the runway. I said it appears your going around and issued traffic and go around procedures. I then asked the reason for the go around and pilot said the traffic was still on the runway. I said he appeared clear and the pilot responded with no if we landed it would have been bad. I said ok Roger and switched him to Departure once he was established on a 190 heading and the Approach Control accepted the hand-off. I didn't write down any information because I was too busy and I don't know the exact location when the Aircraft X started to execute the missed approach in correlation to the Aircraft Y exiting the runway.We had Estimated Departure Clearance Time's (EDCT's) due to weather and being in configuration 4. Once we went to configuration 1; the flood gates opened and we got slammed with arrivals. I don't know what the in-trail was from Center to Approach; but with all the arrivals to XYR it was extremely difficult to get any departures out of XXR and we were in delays for the three hours I was on position. The fix would be to take the departures to XYL but noise abatement procedures are in effect at XApm and we can't depart XY with air carriers. Another factor was the heat and aircraft saving there brakes from overheating and rolling long. I had to send multiple arrivals around to aircraft rolling long on XXL which also stops departures. The midshift should be a single complex operation. Since there is noise abatement off XY we can really only use XX so the tracon should only be arriving XX only. Plus with the limited amount of Controllers on the mid; there are safety concerns scanning four runways that cross or converge with this heavy volume. The workload and complexity is highly increased.