Like with most things, it is partly science and partly art. You have done a nice job handling the “science” piece of sending emails by implementing proper best practices from server config to a general reputation measurement. The other side of the coin is the “art” of improving your sending performance per ISP along with the probability of inbox delivery. This is an intentionally dynamic landscape comprised of filters, ISP discretion, thresholds and unpredictable user behavior (designating you as spam). It requires constant monitoring comprised of tools, people and actions.
Customers of our SMTP service come to us because they don’t have the expertise and/or resources to allocate. For example, you can set up FBLs with each ISP, but you need someone to monitor, interpret complaints and react with a remedy. Or, you send out a marketing email and AOL blocks your IP because they determine the email to be suspect based on any number of factors. Who contacts AOL to unblock your IP. What happens 3 weeks later when it gets blocked again.
So, when you ask what an SMTP service provider does for you, they should be watching your back, providing visibility to your deliveries and advising you on needed actions to maintain a good sender status. If they are simply handling the science or mechanical end, then you will be no better off.
