> Just a week or two ago I had a prospective company tell me that, in order to hire me, I would need to sell them the entirety of my personally-developed IP and dissolve my interest in my own company.

That's unusually awful for an employment agreement. Most employment agreements claim everything developed by the employee on company time and resources, and many claim everything developed while employed, but this is the first time I've ever heard of an employment agreement demanding all past work. Good call on just leaving; not worth even negotiating with a company that tries to pull a stunt like that.