When somebody buys a workshop or an online course I am really hesitant to add them to our mailing list. Then I sign up for something like airtable and I've already got 3 non-transactional emails from them in less than 24 hours. Maybe I should be less shy.
Do one email asking them if they're interested in receiving one email of exclusive content every n weeks. I think this would work. Interesting content and you're telling them you won't over do it.