What is the best way to transition from a senior engineering role in an enterprise software company to a product manager role in a consumer startup?

Avoid #3. Being a product manager is not about having or selling your own ideas. If this is what draws you to product management, you might want to do more research. Being a product manager is about understanding a market, knowing who your customers are inside and out, identifying their pain points, and curating and testing solutions. I say curating (not generating) because rarely is the product manager the idea generator. Ideas should come from everyone within the company. The product manager should work to understand how to evaluate and test solutions. And again, this isn't based on what ideas you think are a good idea. A product manager needs to evaluate ideas from the context of the user and business needs. As for #1 vs. #2, either seem like a good option. If travel is your passion, why not work for a travel company as a software engineer. It gets you closer to your passion and gives you an opportunity to learn more about what product management is like there and if it's really what you want to do.

Deep market understanding is an essential minimum criteria for any Product Manager, so it's great that you have it. Having ideas that are compelling are very good to have too. However, one of the biggest challenges faced by someone moving from a "strong engineering" background into Product Management role, is the "engineer's itch". It is an uncontrollable urge to keep adding features, cool & creative ideas to a Product. Prolific, hard-core engineers do a lot of that, yet it is one of the biggest weakness a Product Manager can have. One needs to train one-self to think less as an engineer and more as a CEO (or investor) to figure out, what is going to give the best return-on-investment. The "engineer's itch" is often so strong, that it clouds your judgement and gives you tunnel vision, s.t. you fail to evaluate the alternative avenues of investment. For this reason, in spite of having deep market understanding and lots of compelling ideas, it is unlikely (but not impossible) that someone would hire you directly as a Product Manager. Which is why, the best route is #2, though no harm in turning route #1 if you think you can move into a Product Mgt (PM != Project Mgt, just to be clear) role. Route #3 may entail the risk of not living up to expectations... not because of your smartness, skill or knowledge, but the lack of experience and that "engineer's itch" thingy! Finally, note that Startups (most often) do not have the deep-pockets and patience to permit folks to "learn on the job", especially when the jobs are largely orthogonal given ones education and experience. There are outliers and amazing engineers who can quickly adapt, but it is a risk.