How do most early stage startups get the feedback they need to iterate and become fundable?

Why would the industry professionals do it for free?

I've found that advice even from industry professionals aren't always so valuable. They might highlight some obvious flaws, but you won't be able to get as good as feedback from actual customers who would be users of your product, and its hard to establish a panel that is able to cover every conceivable solution - especially on the B2B side.

So I'd use it only as an initial litmus, but not for serious feedback.

yes, any feedback is good feedback, especially if these people might be potential users themselves or know a lot about a specific sector.

I think it's a bad idea. The reason you're excited about the feedback you got at the accelerator is exactly because it was from top notch people running an exclusive program, and focusing deeply on a small number of companies they chose to work with. They chose you because they believed in you, your idea, and knew they had the relevant skills and experience to help you.

It's hardly the same as having a "curated crowd" dump their thoughts on you without choosing you, believing in you, or having vested interest in your success.

As an entrepreneur I would trust very few people's advice. There's too much useless advice out there anyway, and I don't see a way for this type of idea not to turn into junk.

My honest opinion. Avoid.