When you want to provide an accessible digital product, when do you that it's done and your product will be fully compliant?

To give a short answer, one of the most common ways is to realize an a11y audit with an expert or a firm specialized in a11y audits. Regarding where you are in the World, you will have to follow some specific rules (WCAG or based on the WCAG). And an overlay will never be enough: https://monsido.com/blog/accessibility-overlays-not-enough.
And without a strong knowledge about these rules and how to test it, it will not be efficient to give you an overview of your a11y compliance.

But what you need to understand is that an accessibility audit doesn't mean anything if you don't try anything after that to fix your issues...