Similarly, my company are 4 years into a rewrite of a cobol mainframe system much simpler than Social Security. Which was going to “take a year” there’s at least 5 years left.

I know the UK benefits system took well over 12 years to build with an programming workforce of over 2000 and I imagine it’s simpler having to support a population one fifth the size of the US.

The Mage Errant series by John Bierce might be of interest to you. Magic and how it works is very much at the centre of this series. There’s some really imaginative world building too!