I understand you plan to make top down, but a 3d game trully, just with view from above?
Question is where you gonna publish it. Will it be android or desktop. If you plan to have a nice graphic then you need learn how use Lighting or LightingPBR shader (it is what are normal, alpha, metal, roughness, glow and more map textures that give very nice result). Also then you gonna know blender or other 3d model tools very well.
But if you gonna make full 2d, then you could simply operate on images.
When you will lunch first test, then you will probably start from “how to load asset”, “how to add asset to be visible in game”(you need learn about scenegraph), “why asset is invisible”(you might need light to see them). Then you will probably need to know how to manipulate this elements.
The best way would be use AbstractControl(extend it with your own methods) and add to Spatial(spatial is scenegraph entity, it can be Node(container), Geometry or anything else)
or just use simpleUpdate(float tpf).
tpf is used to make all actions be independent of frames per second.
for example your character move, you multiply this by tpf(so if someone have 10 frames and other person have 100 frames, it will move same distance)
Years ago I wanted to make a game myself. I searched an open source engine.
I did not know java and my day job is only touching it coding topics.
For me the tutorials worked fine. If you already know java it should be faster and easier for you.
Depending on your background and motivation the only thing lacking is time!
If your life (job) is not depending on making a game this would even be better for you.
I spend a few weeks/months (ok shifts at work) to redo and modify the tutorials. After that I spend a few months /years with writing a concept of a game (ok, again during work).
Finally I started test cases (mainly Appstates) that I hope to use once I make a game. You may find reading this forum being equal to mining (make a game) gold.
I know most people dont have the luxury of time and ressources to do this as a hobby but this is how I have done and learned jme, java and related topics so far.