It sounds like you are on the right track. In today's world, people consume so much content every day that it makes very difficult for businesses to break through that clutter and get their attention.

The big takeaway I think you should continue to focus on is making your message more relatable. If you want to get someone's attention you need to perfectly target their interests, pain points, wants, needs, lifestyle etc... and gear the message to where it feels like you are speaking directly to them.

Now this isn't always easy when you are dealing with lists because you have to be broad. However, one thing you can try to do is get really niche with your segments.

Take that list and try to isolate various groups so that you can hit them with a revised message that is more targeted to them. These "sub" segments could be based off of geographic area, gender, age etc.

With a better understanding of your offering and audience I can certainly provide better examples and other tips here if you would like to book a call!

My approach used a series of single paragraph emails that linked back to relevant industry calls to action. Additionally, we polled prospects to determine their pain points in order of most pressing. The result was increasing a century old trade association's membership net 11% in 10 months.
You have an opportunity to offer your successful approach to the direct mail pros as additional income. Continued success!

A good recipe.

I'd suggest one refinement.

If you can encompass your entire message lucidly in 2 lines great. If it takes more, then write more.

Focusing on framing your message completely, rather than some arbitrary number of lines... may improve your conversions.

A marketing funnel is a set of steps a visitor needs to go through to become a customer. It is a system designed to attract and convert customers to your business. You can measure and track these steps in your marketing funnel. The point of having a marketing funnel is to set up an automated system that is measurable at any point of the customer journey. When prospects reach the bottom of your marketing funnel, the chance of becoming customers increase.
The first step at the top of your marketing funnel is Awareness. Awareness is where your customers are coming from. On the first hand, this allows your prospects to feel how it's like to do business with you if you're doing B2B email marketing. Once interest generated, your subscribers will start learning about your business and what you have to offer. You can do so with targeted content, personalized to what attracted your subscribers in the first place. This stage of your marketing funnel consists of timely follow-ups to nurture your leads and engage customers. Apply various touch points that can help move your customer along in your marketing funnel. Always remember to have a clear call to action after each touch point so your leads know what the next steps are. Now that you nurtured your subscribers, they are now at the bottom of the funnel and considered sales qualified leads. Share existing customer reviews, benefits, and explaining the use of your product.
You have delivered the value you promised and positioned yourself as an authority. Now in your third email, you go for the sales copy. You could use a structured pitch in which you can connect with the customer's problem. Or you can state the Why, What, and How of your offered product/service. For example, Leadfeeder sends this final email to trial users. For that, your emails should be short, catchy, and direct. It helps you reach out to your leads immediately when they are most interested in your business. You can use triggered emails for your new product announcement emails.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath