my point is: Buying an ETF WITH bitcoin and NOT with Fiat money. Current state is that I have to change my btc into fiat to buy an ETF
There is no legal route to do so yet, but that time may come eventually. I am not aware of an unregistered platform allowing you to invest in legacy instruments using Bitcoin, but I would stay far away from them.
Regulators are very strict in this field and whenever they shut down such platforms you'll likely lose all your funds, which they then auction off for their own benefit. That's how things go here.
I'm not sure what ETF you plan to buy, but it makes more sense to hold your Bitcoin now the sentiment is improving on a weekly basis. Also, the potential returns through an ETF are peanuts compared the the returns Bitcoin yields.