The career move feels right.
That's exactly when it's most dangerous.

This is a decision that already has momentum behind it. You have been thinking about it for weeks, possibly months. The fact that you are running it through a structured analysis suggests you suspect something: either the case for doing it is not as solid as it feels, or the case against it is stronger than you are letting yourself admit.

Career decisions fail in predictable ways that are invisible from inside them. The most dangerous version is not the move that blows up immediately — that at least gives you information fast. It is the move that works out okay, just never as well as you imagined, and locks up three years of your life in a direction that quietly was not right. That version gets called a learning experience rather than a mistake. It is the failure mode worth examining hardest before you commit.

The specific move here — leaving a stable product management role for a Head of Product position at an early-stage startup at a 15% pay cut with equity — has several structural variables that are independent of whether the role itself is good. The company's funding runway, the actual scope of the role versus the title on offer, the founder relationship, and whether the equity is worth the paper it is printed on all matter more than the salary delta in isolation. None of those are visible on the surface of this decision as presented.

The side project generating £800 a month is sitting at the edge of this analysis in a way that suggests it may be the real decision being avoided. It is included here as context but has not been examined as an option. That asymmetry is worth naming before anything else.

The framing of this as a binary — take the startup role or stay — is obscuring the real question, which is whether you have correctly assessed what you are moving toward versus what you are moving away from. People who make career moves primarily because they are unhappy where they are tend to recreate the same dynamics somewhere else. People who move because they have identified something specific they want to build or learn tend to do better. Which one is this decision?

The £800 a month side project deserves a direct conversation rather than a mention in the context field. You included it. That means it is on your mind. If you went all-in on it, what would it realistically take to replace your current salary? If the answer is a long time and a lot of things going right, that is useful information. If the answer is 18 months of focused work and a clear path, that changes the shape of this decision entirely. The fact that you are considering a pay cut for a startup role suggests you have some tolerance for financial risk — it is worth asking whether that risk tolerance is better deployed on your own thing.

On the startup role: the equity offer needs scrutiny before this decision is final. Percentage ownership, number of shares, the current valuation, whether there has been any dilution from earlier rounds, and the liquidation preference structure all determine whether the equity is meaningful upside or a consolation prize attached to a pay cut. If you have not seen a cap table and asked direct questions about the funding situation, you are making a financial decision with incomplete information.

The founder relationship matters more than almost anything else in a Head of Product role at an early stage company. You will be the product team. That means dealing with the founder's product instincts, their communication style, and their risk tolerance daily, frequently in disagreement. The question worth answering before you accept is not whether you like them but whether you know what they are like when they are wrong and someone is telling them so.