I was in a similar place last year and went the Mailchimp route. 8k subscribers cost $75 a month. Instead I’d recommend using Amazon’s AWS SES service. The first 62,000 emails a month are free, and the rest cost just $0.10 per 1k emails. Pair that with a decent Wordpress-based Mail app and you’ll have a fine email marketing operating for 1/4 of the price.