@Trazo -
First, let's get straight on the term "whole arm movement". This term specifically refers to writing with the whole arm lifted off the table (or sliding freely on top of the table). Whole arm movement was recommended in Spencerian/Ornamental books for writing capitals, but never recommended in any of the Business Penmanship books as far as I know.
I would assume that you are actually referring to "muscular movement", which is writing with the forearm planted on the edge of the table. Just want to get that straight so I know exactly how you are using your arm to write. A misconception here would be costly.
As far as "mastering" muscular movement, I've gained a certain level of control with the arm that you might be after, but I'm far from mastering it. It might be helpful to look at my movement from one of my earliest videos on Instagram vs something more recent.
From Aug 2017 (10 months into my practice):
https://www.instagram.com/p/BXiQVWflfOb/And from September 2020 (~4 years into my practice):
https://www.instagram.com/p/CE9V5ULDk-w/Hopefully you can see the differences between these two samples. Unfortunately, I don't have video from when I first started, but I can assure you it was very, very bad.
I do have this picture which features three samples of my work from early Feb 2017 (3 months into my practice), 2019, and 2020:
As you can see I was severely struggling, even after 3 months of pretty much daily practice. Also feel like I've made the most progress this past year since my first 1-1.5 years of writing.
The point is that it takes time. Everyone I know that has developed a high level of control with muscular movement has spent countless hours working on it. That said, I always reserve the possibility that there is a better, faster way to develop control that no one has figured out yet.
Overall, if you've only spent a few months of daily practice, I wouldn't expect to feel like you've accomplished much or like there is light at the end of the tunnel. I remember writing to Marcus Carlini with same general question that you are asking now and he basically said, "Keep working on developing your arm, keep studying the forms, and control will come eventually". That gave me confidence to keep going. Hopefully this does the same for you.