Your budget wouldn't even buy much of a frame improvement, let alone a bunch of new parts. A new Chorus 12 group could be had for your budget. Of course lower level groupsets would cost less.

Look for sales on leftover bikes from a year or two ago. The wheels won't be as good as the Zondas, so swap them over.

I've got a 2017 Colnago C-RS that I got real cheap as a 105 equipped bike last year. I sold all of the parts and built it up with 2020 Chorus 12 speed and Zondas for about $3000. These days, that's cheap.

I also have a 2004 LOOK KG461 that's got all new Chorus 12 and Zondas. It's not as stiff as the Colnago, but only weighs about 150 grams more. It makes a nice winter/trainer bike.