Curious what other people have done here before about email sequencing/newsletter - has anybody integrated their Solidus install with any Convertkit/Drip/Mailchimp/etc. to the point of tracking who has/hasn't purchased certain products?

Yeah, that's very common. Most of those integrations are somewhat custom. There's different ways to do it, but usually people make a post purchase job that subscribes the customer to the mailing list with whatever additional data ("merge fields" in Mailchimp parlance) if the customer opted in.