If NAT already obfuscates my private addresses through translation, how would a potential adversary connect to anything beyond it?

Not an expert but they can try compromising another device on your LAN as a proxy to your rig. Maybe pawn your router and have it open up random ports too. So per-device firewall is defense in depth.

What "good" would my public IP do for a hacker if I have no ports forwarded?

Is a firewall a second line of defense just in case I execute malware that starts forwarding ports?

Malware doesn't need forwarded ports to the internet to function. It can just download a script to a compromised device and wreck havoc on LAN. So if you properly segment your devices and utilize endpoint firewalls it can limit the blast radius and does some detection stuff

Edit: Don't think of NAT as proper firewall, it's just an easy way to share addresses via your router/modem. Your ISP's devices often block inbound connections from the internet by default but that's a firewall configuration, not a NAT