Ideas below - but if you can't find the source then please take the car to any competent mechanic. Concern is that a loud banging means something in the suspension is very loose and needs repair to be a safe vehicle. This is not a dealer-only problem. And, diagnostic charge will often be waved if they quickly find the issue and you have the shop do the repair.
- Broken strut spring can make this noise and sometimes be harder than you think to see because the crack is often near the very top or bottom
- Lower control arm bushings - also hard for you to test because they are hard to see and you need near to full car weight to load them. A shop with a lift, experience and possibly a long pry bar will be able to confirm the issue.
- Lower control arm has a ball joint that could be loose enough to clank. This you should be able to test with the car supported by jack stands and a long pry bar
- Steering rack has both inner and outer tie rods each with a ball joint that can become worn out and loose - unusual to be a large clunk, but possible
- Sway bar bushings and sway bar end links - though it sounds like you know what these are and have confirmed they are not loose
Just my opinion: Diagnostic fees are fair. A professional shop needs to protect itself from customers who use them exclusively to diagnose but drive the car away to fix it themselves or have a friend do the repair. We all deserve to be paid for our time and expertise.