Happy you chose to learn C. That's exactly what I would recommend.

If you are so inclined, I'd look into how to write your own home-grown version of Objective-C—given that valid C is valid Objective-C.

Because then you would learn Smalltalk and the whole ladder of it all! Smalltalk is far and away one of the greatest joys on programming. :) http://objective.st

The "Gang of Four" explains the patterns in Smalltalk and in terms of entrepreneurship—Patrick Collison's first startup was written in Smalltalk and I think early introduction to it leads to the clean design and success you see coming from Stripe.

You might find it interesting to write some native macOS or Windows software (ie using the native GUI library). If you've mostly focused on web software you might find the different strengths and constraints of truly native code inspiring.