> I see constantly banners on sites that set tracking cookies by default, and delete them if you reject them in the banner (or even worse, not delete them at all!) – this is not compliant as the cookies were set before consent was given

Depends on what you consider to be "cookies were set". I think it's a valid argument that cookies aren't set until a "Set-Cookie" HTTP header is sent to the server. The banner is just a form to decide whether or not to set the cookies prior to actually doing so. The banner switches aren't the cookies themselves.