What stage in the product lifecycle is best for a product launch?

I think when you have your first MVP

There is no definitive answer to this question as it depends on several factors, including the type of product, the market, and the company's objectives. However, many companies launch new products during the early stages of the product lifecycle, such as the introduction or growth phases. This allows them to take advantage of a new product's buzz and get it in front of potential customers before the competition.

at least the stage where you can successfully collect emails on your website, whether for an idea in development, or already developed and looking for beta testers, etc.

Generally, as early as possible. Simply because introducing potential users to your product as early as possible, will help you uncover things you haven't even thought about. Be it; - users inform you that some absolutely crucial feature is missing without which they don't want to use the product - users show absolutely no interest in a feature you were planning on building and now you realise you don't actually need it - users asking questions about how to use a feature and you being able to adjust your product to improve the user experience It's also worth adding that often some of your earliest users, will become some of your most loyal as they feel like they have directly guided what the product became and now feel invested into it.