Where can I find detailed historical breakdowns of U.S. lightning fatalities on a year-by-year basis?
For a comprehensive literature review of sources of lightning fatalities data you can check the paper "Striking Back: An Assessment of Lightning related Fatality and Injury Risk in Canada", it covers US and Canadian sources. But if you want to know which one is the largest and arguably the best. That would be NOAA's Storm Database. The data you can access trough the online interface is quite limited, so I would recommend to go to the Bulk Data Download area, and get the original spreadsheets. You will see 3 files per year: Locations, Fatalities and Details, from 1950 to 2017. I would recommend to download the "Details" files, as it also have information about the location (Lat/Lon) and number of deaths and injuries, and a short narrative of the event.
It is a large table, as an example, I took the following screenshot of the most important data columns for all the lightning events with casualties in 2009:



You can download the Fatalities file if you want more info abut the age, gender and location of the victim. The relevant fields look like this: