Best of luck with this. I thought you were doing a server in C though?
For GPU rendering, you can probably check vs runelite. The way they do it is kinda jank but it should get you a quick and dirty rendering of the game world.
Also, regarding 5. Migrate from old and deprecated technology (remove applet, look at migrating to jfx).
I am in the process of changing the window management/keyboard/mouse classes to use GLFW.