I thought I had read sixt generally does 2-3 times the rental amount? In this case we are talking 20x which is quite a bit more. Europcar for example charged me a 500 euro deposit last rental on a 240 euro rental which seems reasonable. I also declined insurance in that case.

It was irritating as he didn't ask for confirmation before putting it on the card. It put me over my limit but oddly went through. I would have preferred to use another card and avoided that issue altogether.