For the boss you should make it sort of similar to Necrodeus (from Kirby Mass Attack). If you don't know what he looks like just search him up. I don't know how many hearts he would have, so I'll leave that up to you.
I also have a question:
Are you going to make a normal difficulty which is easy difficulty but when you smelt nether star, you only get one nether star gem (you still use sticks and ores) or you use nether star (not the gem) only but you still use sticks and ores? Please, I'd love it if you use this idea.
This would be amazing, but should only happen after a Wither has been defeated.
Agreed. Maybe you'd have to kill a certain amount of withers, or sacrifice some nether stars to make them start falling, and maybe you only get 1 every few days? Maybe you'd need to make a beacon near a nether portal. Or maybe the sword could have a chance to drop nether stars from other mobs, and the pickaxe have a chance to drop it from ores, maybe redstone or diamonds. Or maybe the nether star pickaxe has a chance to drop it from every stone block? To simulate rarely finding an ore.