I know that you said you don't want a premade kit and following instructions and this is *close* to that... but I think it might still be a great fit.

Mark Rober has a new product where they ship a new robot every 2 months. They give you the basic instructions on how to build/program it but the idea is that you take that knowledge and then expand on it yourself by adding features. My daughter is still a little too young for it so I haven't used it personally. The biggest issue is that it is a subscription and not a one time purchase.

Here is the link: https://www.crunchlabs.com/products/hack-pack-subscription

And here is a brief video explaining how it expands beyond the normal "premade robot kit." https://www.youtube.com/watch?v=TtdOdUi9b_s