What tools do you use for your email campaigns?

I use SendGrid for automatically sending users a welcome message when they sign up on the https://lyrist.app mobile app

We used Sendinblue before but later went to HubSpot. Its easier to have all your customer data in one place.

I'm curious to know if any of you have had a waitlist, how do you go about sending emails to notify customers with access? Is there a way to split up customers in different groups to receive the access? (ex. 100 get access Monday & another 200 get access Wednesday)