When first getting this wonderful machine, it does seem overwhelming. I see that you have the Janome Apps and I am hoping that does include Diane and Jim’s App named My 15000. This is the best way to learn everything about this machine. You can first read the explanation and then watch the video. If need be then go to your machine with the written explanation in front of you and then try to work through that step. You can always watch the video again.
Part of that app bundle is the FootBook which also is very helpful for the use of some of the special feet and accessories like the quilt binder...( but not right away)
And if you are a quilter, you will love the Quilt in the Hoop app... but here again learn the machine first because there are things to learn with this app.
As to the size of the USB, I have been using a 2GB and no larger! If you were to load the entire drive with designs you would be surprised how many that is. I periodically will empty mine to folders on my computer so that I can easily go back and find one design I love. And if you drag and drop files to folders or to the Embf on the USB you will find that a very easy method. I personally can’t use the WiFi on the machine as I live in the boonies with very slow iffy WiFi. and i have used this 2GB flash drive method for the 5 yrs I have had my machine and am very happy with it. I did the same when I had my 11000.
I hope this has helped a tad. Sorry I didn’t take time to write sooner to help you. Somehow time gets away too quickly.
Pat in Md.