Cloudflare provides very rich data and analytics for websites. But a very important data is missing:
520 Errors Log
You need a user reporting you to learn about this error.
Other important issue is when I disable Cloudflare Proxy I never see 520 error or any other error message on Google crawler logs. These 520 errors are the only reason that I do not use CloudFlare services.
I see you are already participating in this thread which is a product request for logging similar errors, so probably best to keep it there. Product requests with a large number of votes are likely to get attention.
You can get logs on the Enterprise plan, or with an app like logflare.app, if that would help.