How would you repair this?

As a professional welded by trade.
Drilling the crack in sheet metal does nothing. It’s sheet metal. Not enough grain structure to carry a crack further. The crack started by loose seat or phat az in the seat rocking back and forth and created the stress. Honestly weld it up with e-70s welding wire (mig machine) and call it a day. If you remove the reinforcement plate and install a larger plate (more surface area) even better.
Nothing to loose sleep over. Easy repair.