I agree to see what the other positions pay. Phlebotomist seems more promising than EKG tech. Probably not as much money as LPN, but likely more than CNA, and easier on your back.
I think you are wise to be thinking about doing something other than CNA work, both because of the financials but also because very few people are able to meet the physical demands of CNA work all the way to retirement age. At 40 you're probably okay, at 50 you'll be slowing down, and by 60 you'll be at great risk for injury, if you make it that long.
Is EKG tech an in-demand job? Not being snarky. In my hospital the CNAs all do EKGs. It's really not that hard to learn where to put the 12 leads. The doctor orders an EKG, usually STAT, and the CNA taking care of the patient that day does it. I'm trying to imagine the context in which someone's job would just be to do EKGs, and I'm coming up short.
Your child is 6, so I'm assuming s/he is in school, so you should have some time for classes and study. Yes, nursing school (LPN or RN) can be challenging, but that's WHY it's a path to better pay. It it didn't require time or effort, anyone could do it.
Since you're 40, you've probably been out of school for a long time. There will likely be a learning curve as you get back into the swing of studying. However, lots of times mature students who go back to school do better than they did when they were younger. By 40 most people have better focus; they are highly motivated to meet their goals, and have better self-knowledge (i.e. they know what they need to do to get things done).
If you can get into a reputable LPN school (not super expensive, good NCLEX pass rate), and you make studying a priority, I think becoming an LPN is a good idea.