Re “Productivity equiation” from Ali Abdaal: Productivity = Useful output / time x f (where f is the ‘fun factor’), but treating f as constant is wrong - even fun projects can get tiresome . A good measure might be instead an inverse 'tediousness' factor, e.g. how tedious are the worst expected tedious parts of the project (like the 'fun' bottleneck) and make sure you don't get stuck when you reach those points.
Also fun project tend to be novel, but novel project have the highest amount of unexpectedness, so its good to decide on a few 'novel' things and let the rest of the project be based on stuff you know well.