He estimates that he talks or texts, on average, with each CEO in his portfolio three or four times a week. He visits potential customers with Jörg Lamprecht, CEO of Dedrone, a 75-employee drone-security firm. "It's not about the investments," Mr. Chambers says about his portfolio. "I want to first be a strategic partner with the CEO, where I am his or her most trusted adviser, mentor--a coach." Mr. Chambers is part of a wave of big-time CEOs and executives who, instead of more conventional retirements of just sitting on a couple of big-company boards, are embarking on second chapters as guiding lights on the startup scene.