I'm not a copywriting expert however I can shall some basics that have served me well for several decades. My first lesson was the importance of understanding my target audience.
I like to think of myself as a fisherman. Knowing what species of fish is important. I'm not looking for fighting fish, sharks, or sardines, for that matter. Salmon, Trout and catfish are tasty and fit on my plate. Trout and salmon are exclusive to fresh water, so I don't need to fish in deep open ocean waters. And the bait and hooks are more specific to their liking.
Fishing for deals using paper (direct response mail) is similar. I want to use the right bait for the right species, and repel the little stinkers that steal my bait (unmotivated sellers).
Personally, I like working with sellers with big problems that I have experience in solving. I started in the late 1970's doing equity purchase deals. Using paper default lists I'd mail to folks with what appeared to be good equity prospects and qualify them when they called.
That's where you are now, trying to handle the lead calls your letters produce and balance the time you spend handling these calls with your non-real estate duties. Since I don't like being rushed and don't like wasting my time with unrealistic peolpe, the challenge is to design a marketing campaign that meshes well with a sales conversion process which attracts the sellers I want and resells those I don't want. Some call this "self-selecting."
My most effective strategy has been to position myself as an expert in specific areas (probate, foreclosure, title), market to experts (attorneys and other trusted advisors) and use well-placed articles and other value added information within the view of those who would benefit from the knowledge.
In letters, I like to spell out the types of situations or scenarios that are my forte in problem solving. I tell would-be clients or sellers that I'm not not a price leader and have no interest in competing with people and companies who put themselves out on the cheap. After all, cheap is cheap. If everyone was concerned most with price point, we'd all be driving Smart cars (or Yugo's).
Another way of approaching thesis to list out the U.S.P.'s that would only appeal to the sellers that you wish to do business with. As an example, you could go after people who've recently had a negative experience with another real estate pro and market to them with an appropriate USP based on trust or a "Plan B" offer. Match the proposition to the problem you intend to solve and make the reader understand on an emotional level that you are unique in your understanding of the issue.
And don't be afraid of blowing off time wasters early. You can do this by both educating your reader/target and give them examples (or even testimonials) of the species of how you've helped others. And examples of who should NOT contact you.
Sounds like you're off to a good start.