Your customers are YOUR customers so you can contact them as long as you don't send promos or offers through Etsy convo as this is not allowed. Ideally, every time you get a new customer you need to ask them if they would like to go on your email list and giving them 15% off their next order as an incentive to do so typically works.
As for previous customers, particularly ones that left you good reviews and are repeat customers, you could email them (do NOT use Etsy convo, use email) with an offer and then state in the email, "If you do not wish to receive further emails from us, please let us know". I definitely would not email more than quarterly.
