Legalization puts barriers in place for sex workers, especially vulnerable ones and creates the illegal tier / market. Full decriminalization is the safest option for workers as they are all equal under the law regardless if they have registered.
You're advocating for the Nordic Model, which was a colossal failure. Sex workers have no protections, they're harassed by police, evicted, etc. Legalization opens the path to regulation and protection that otherwise isn't afforded.
The 'illegal tier' already exists, it's a farce to suggest that legalization "creates" it.
yes, this is a flaw in some legalization regulations. but only decriminalized lacks any consumer protection. I agree with you about optional registration, as least then consumers would have a choice about assurances of certain aspects.