Until someone has used it, a prototype is just an opinion with a nice layout. Testing it takes no lab and no budget. You need five people, half an hour with each, and the discipline to stay out of the way.
Why five. The first sessions reveal most of the blockers, and later ones confirm them without adding much. Past that point, each session costs the same and teaches less. If your users fall into two very different groups, plan five sessions per group.
What to ask. Give a mission, not a request for opinions. "Find last week's order and tell me when it will arrive" teaches far more than "do you like this screen". People stay polite about opinions. Faced with a mission, they show you what gets in their way.
How to behave. Stay quiet, even while the person is searching. If you have to explain a screen, that is already a finding. Write down the hesitations, the missed clicks, and the moment they ask for help.
How to wrap up. A blocker seen once is an anecdote; seen three times, it is a problem to fix. Testers pin their comments to the section in question, and you send those comments to the AI one at a time or all at once. The next round is ready within the hour.