I think the 235 is probably the biggest factor in your understeer. Being tire limited would also help explain why the shock settings up front aren't making a significant difference for you anymore. You've got them in the ballpark and there's only so much grip so small tuning changes aren't coming through.
But the fact that you've gotten that close to a more modern car with an experienced driver should tell you you're on the right track for your upgrades. You can make the car understeer by going too stiff in the front, so, unless you're still noticing a body roll issue in the front I think my next upgrade would be more tire, not a larger sway bar.