Don't confuse the separate things here. A domain name is a website address. It can be but isn't necessarily a trademark, which designates the source of goods or services. "Tag lines," or slogans or phrases, can also be trademarks, just like Nike's famous phrase "Just Do It."
Apparently you haven't started your business yet, so all you own is a domain name with 3 extensions. That's it. You've spent $50 and have invested little else so far except maybe some outlines and plans of what you want to do in the near (but still) future.
Trademarks are acquired by USE, not by application with or registration by the USPTO. Now you've discovered competitors' uses, in apparently at least one of the same international TM classes that you were interested in, although that's not clear either since you haven't specified class 41 or any others, and it's not clear that any of the others actually would compete with what you plan to provide.
Before you make any more decisions about what you plan to do and how to best do it, now is the time to see a lawyer about helping you plan the rollout and protection of these educational services and this book series, and whatever other goods and services you plan to provide. You may have to start from scratch with a new domain name, but that's better than doing that after you've invested a lot more time, effort and money.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Registration of a trademark by another company does not necessarily trump your potential rights to use the trademark. Common law trademark rights can also exist, and in some instances, can supercede a registered trademark. The important inquiries to be made are whether you used the trademarks in commerce prior to the other users, and whether the trademarks are likely to cause confusion (i.e. whether the trademarks involve the same line of business, the identical names, etc.) Therefore, I would need to know more facts about when you began using the domain name (not just when you registered the domain name), the nature of the usage, whether your usage predates the usage of the other tag lines you referenced, and whether confusion between the marks would be likely. I would suggest you consult an attorney to take you through this analysis and to determine what rights you may have in those domain names.
The answer, of course, is that it will depend on the precise facts of your past use, current use and projected use and the use that has already been made by others. You need to make sure that if you go forward with plans to use the tag line, you are not creating the impression that the book series is somehow associated with or sponsored or endorsed by those who are conducting the educational conferences. You would be well advised to consider alternative names or tag lines and, certainly, to consult with counsel before proceeding.
Disclaimer: This answer does not establish an attorney-client relationship or constitute legal advice. It is for general informational purposes only.