> However, I still think waterfall is the only approach when trying to design a hardware product.

You can still use some agile ideas when working in electronics, thought it might look quite differently than in software.

For example, if you are a startup and your plan is to create a super duper wireless speaker with bluetooth, wifi and AI, you could first build a wireless speaker with only bluetooth, and sell that. That way you build experience with producing speakers, sales channels, manufacturing, sourcing etc., and build a customer base.

Then you could ask your customer base if they'd prefer wifi or AI in the next model, and iterate accordingly.

This is not the same as doing one-week sprints in software, but it does iterate that you can start small, build and sell small, viable products, and work your way up to the intended product.