Just catching up on this thread. What kind of portraits are you leaning toward, tight headshots or more environmental stuff with Chicago architecture in the background? Before dropping cash, consider these alternatives:

• Sigma 35mm f/2 DG DN Contemporary: Great build and optics, though a bit heavier.
• Sony FE 50mm f/1.8: Super cheap, but autofocus can hunt in low light. Let me know your portrait style so I can narrow it down.

Honestly dropped way too much money testing glass over the years, so I feel your pain after buying that a7iv. Since you are on a strict 600 dollar budget and doing Chicago street stuff, be careful not to fall for the hype on overly expensive G-Master lenses right away. I would suggest looking at these options instead:

• Sony FE 35mm f/1.8
• Usually sits right around your budget brand new, super light, and sharp enough for low light.
• Sigma 35mm f/2 DG DN Contemporary
• Built like a tank with metal housing, though the aperture ring takes some getting used to. Make sure to check local classifieds too cause you can easily find a used Sony FE 55mm f/1.8 ZA Carl Zeiss Sonnar T* for well under 400 bucks if you want crazy sharp portraits instead. Just watch out for dust on used copies!

TL;DR: Just grab a fast 35mm prime. In my experience shooting streets here in the city, the 35mm focal length is simply unmatched for versatility. Over the years I've tried many different setups, but a standard 35 always stays glued to the body. It gives you enough width for downtown architecture while still letting you isolate subjects for casual portraits without feeling cramped. Go with Sigma or Sony, you really cant go wrong.