That's a nice list you got. I would not remove anything and do not feel you have to add anything essential. But, perhaps, you can skim a popular book first and see for yourself what it takes. Perhaps you've done that already. (Just in case, there's a nicely written introduction by Einstein himself Relativity, Crown Publishers, 1961.) Tensor analysis and differential geometry rightly top your list. The rest are the rangs on the ladder that leads up there.