> "An IP lawyer told me even a basic patent review would cost thousands"
What is claimed that you infringed? From that you can start by searching yourself if you really infringed their patents. Look at the claims, not at the main introductory text. Good patents are novel, non-obvious and useful.
If you feel you have infringed their patents, then you can search for prior art to their patents. But for you to have some chance to convince a judge who has no knowledge of the domain, any prior art has to be pretty obvious.
What is claimed that you infringed? From that you can start by searching yourself if you really infringed their patents. Look at the claims, not at the main introductory text. Good patents are novel, non-obvious and useful.
If you feel you have infringed their patents, then you can search for prior art to their patents. But for you to have some chance to convince a judge who has no knowledge of the domain, any prior art has to be pretty obvious.