This is not a Thunderbird issue, and you are well advised to work with a legitimate mailing service. Your mail is classified as spam or not spam by the recipient, so it's not your call. Generally, email providers do not like any of this because it can get their servers blacklisted. And even mailchimp can't protect you ( I once administered a large mailing list with mailchimp, with all subscribers having done two-step subscriptions, yet for approx 5 months almost 1/3 of recipients did not receive the mailings - which came from a government organization. )

Being an author myself, I respect your desire to reach people, but we cannot judge what is and isn't spam, and providing assistance in mass mailings would put us in that situation. But I wish you well. (And this is a personal opinion, not necessarily what the big wigs at Thunderbird might do.)