When to apply for trademark?

Your approach is sensible for the U.S. where trademark rights accrue from use of the mark. So as soon as you are offering the goods with the mark in U.S. Commerce, you can already assert rights in the mark against subsequent users/registrants of the mark in the U.S.
However, trademark rights are territorial and differ for each country so you need to consult with a trademark attorney in each country where you intend to claim trademark rights. Most countries actually require registration of the mark to acquire trademark rights and the first to file a trademark application may be the superior claimant to the mark.
This is particularly true for countries like China and Russia. In fact , your approach would be completely insensible and ill-advised for China where someone could view your mark in use in China (or elsewhere) and beat you to the Chinese trademark office to register your mark and then basically extort you to reclaim your rights in the mark in China.
Really, the best approach to ensure that your investment in branding will not be wasted because of a potential conflict with another company is to immediately, before undertaking any investment in the brand, hire a trademark attorney to properly search the relevant market(s) for the availability of the mark and to assess any potential risks with the mark. That is what we, from a legal standpoint, consider to be the most important and necessary initial branding steps - more important than registration of the mark in the U.S. for the reason explained above. As I understand it, in the scope of all the initial business start-up costs, trademark registration and branding tend to be lower cost items compared with more valuable intellectual property like patents and inventions.