you can use zapier or integromat to sync your ghost members with an esp like converkit or mailerlite.
if you have many you subs it could become a hassle to keep everything in sync. remember, the automation system needs to sub and unsub someone from both services.
i posted 10 min ago an article about using ghost as an esp. in there, i say one of the reasons why i kept using ghost for newsletter is that emails were already reaching the inbox while using convertkit was still going to promotions.
so, beware that when you make the change it’s better to let people know and ask them to move you back to inbox. and even then, open rates could suffer