Informationen zu Mailchimp-Tarifen | Mailchimp - it sounds like you are getting close to some of the limits here.
Pricing and Plans | SendGrid - eventually limited to 100 emails per day for free, but their prices look pretty reasonable for your use case (comparable to a shared hosting plan and cheaper than Mailchimp).
If I were setting this up on ClassicPress today, I would probably use https://wpforms.com/fix-wordpress-email-notifications-with-sendgrid/ to get WordPress able to send its default emails through Sendgrid. Then I would look into a plugin like this to get the subscriptions working (I do not know their stance on ClassicPress compatibility, but we’re also not planning to change any mail-related functionality in the core software):
- Icegram Express formerly known as Email Subscribers – The Ultimate Email Marketing & Automation Plugin for WordPress – WordPress plugin | WordPress.org
- Newsletter – Send awesome emails from WordPress – WordPress plugin | WordPress.org
- more options: Search Results for “subscription email” | WordPress.org
(The thinking behind plugging together a couple of pieces like this: you probably want something that works with WordPress’s built-in wp_mail function, and then you can separately get this part working how you need it to, rather than a plugin that tries to do all of the related tasks at once.)