For example, someone told me it's nonsense to hold btc and ltc, because ltc follows btc pumps/dips and I should only hold 1 of them.
Should I consider adding a privacy coin?
Or even more "platform" coins like eth?
This is bullshit, pretty much every coin follows btc. Except if some coin is pumped or has some major update. You can make sure of this by going on coinmarketcap and see what happens now, when btc is dropping.