One thing that Bob may want to look into, is also having local trade organisations participate. Their involvement would be very minimal, but very useful.

If you create a smart contract that allows them to add a signed record to the blockchain that effectively says that they vouch for that tradesperson in a particular set of disciplines until a certain (renewal) date, that would allow your application, and your userbase, to be sure that their tradesperson is adequately licensed without having to check credentials that may be easy to forge or may be difficult for the layperson to grok.

Revocation could be handled using a similar record.

This does mean that there's influence from a third party, which does sound like a bit of centralisation, but it's a third party with a vested interest in generally keeping things legit, and having an independent third party vouch for safety, compliance and other certifications can't hurt consumer confidence.

Either way, it might add to the benefits you're providing your customers.

It'd be good to use an existing framework or create something general purpose for other folks looking to implement similar stuff in different areas or industries.

Happy to chat more about it if anyone's interested. Hit me up.