Any advice on how to delay the rust on my gen 2 Prius?

Looks just like mine when it finally gave out. I replaced the rear subframe with a used one. They're not that expensive but shipping kills you. The part was I believe 100 and came with a complete assembly with the sway bar and hubs. Shipping was like another 300. To replace the rear subframe, it was around 4.5h in labor.

I did some upgrades including the bushings when it was swapped.

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Same as mine. Opened up the rust on one side and had it repaired for $400. Under the rockers are plastic covers held in by large clips. Pop the clips and remove plastic covers. Clip holes are open to the inside of rockers. Spray P'Blaster Surface Shield into the openings forward and back through each opening. For the rear sway bar, get a hand blaster from Harbor Freight, you'll be surprised how it cleans off the flaking rust, then brush with Corroseal rust convertor or use Surface Shield.

P'Blaster Surface Shield same formulation as Fluid Film and cheaper, also not as viscous as Fluid Film.
Corroseal at Rodda Paint, Gemplers Rust Convertor same product looks to be private labelled. Lot cheaper than half pint bottles of other convertors. $30 quart, amazed how a little goes along way.

Corroseal rust convertor video
Blaster 12 oz. Long-Lasting Surface Shield Rust and Corrosion Protectant, Lubricant Spray 16-SS - The Home Depot
18 oz. Portable Sand Blaster Gun Kit