I am by no means an expert on Spencerian and I hope that someone who is - will add to the conversation.
I can share a few things that I learned from a workshop with Mike Sull as well a few trips to the IAMPETH conference.
I can't see any details on your scans. Because it looks like you are very consistent, it might be more helpful to post a scan that shows just one example of each word in detail.
https://www.iampeth.com/pdf/lessons-ornamental-penmanship-zanerThis PDF at the IAMPETH website has some samples in the back that are quite lovely. I know what you mean about the penmanship in the books not being as lovely as what you see online. Because the books have to teach the basic building blocks, they are not going to be as lovely. However, those basic building blocks are essential. It's like learning the scales in music. Putting in time on the basics is essential. The pretty stuff comes a little later.
I know that Mike Sull encourages people to spend a fair amount of time on developing rhythm. If your book has cross-drills, it would probably help a lot for you to do some.
I would recommend that you spend some time writing several lines of text all at once, rather than repeating one word over and over. If you are going to be combining your lettering with your artwork, you will probably have text rather than just individual words. Maybe not. But it would be appropriate to practice what you are envisioning on your artwork from the beginning. If your plan was to address envelopes, then it would make sense to spend more time practicing addresses rather than writing long passages of text.
I look forward to seeing some details of your writing. It would be nice for you to post it right next to the same words that are in your book. I have a hard time critiquing when I have no idea what the person is looking at. There are several variations of Spencerian so it's helpful to know exactly which one you are working on.
The IAMPETH website (Lessons) has Spencerian Ladies Hand which is a really beautiful variation. There are two versions.
The one on the left has some very pretty caps that are easier than the traditional caps.
https://www.iampeth.com/lesson/spencerian/spencerian-ladies-handMany teachers agree that it can be very helpful to do some practice in pencil. It helps with developing a rhythm because you don't have to deal with the nib and ink - you can just focus on the shapes. Finding the right paper, ink, nib combination is really helpful. I spent almost 10 years being very disinterested in pointed nibs - and I didn't know why. Then - at the workshop with Mike Sull, when he had found the Nikko G nibs to be very beginner friendly with McCaffrey's ink and Rhodia paper - I had a complete turnaround. Pointed pen became my favorite. I know a lot of people do not like how stiff the Nikko G nibs are - but for beginners - if you have McCaffrey's ink (or walnut) you can get really nice hairlines. I can't get satisfactory hairlines on any of the printer papers. So...you might have to try some different nibs-inks-papers before you find the ones that give you the results you are looking for.