this is only partially true: if you want to fully optimize for expected returns, you will pick a stock only portfolio with bias towards small, value and emerging markets stocks (according to the modern portfolio theory) (I did that to some extent). BUT, mixing in a bit of commodities, real estate and bonds can reduce volatility by a lot and only marginally reduced expected returns. no free lunch between risk & return! check out the great portfolio finder and mind the efficient frontier graph.
this “risk-free” part currently (refferring to the Niedrigzinsumfeld" will give you returns of about -0.7%, i.e. negative returns. that is why people on this forum agreed to preferring cash over bonds, at least until their yield is positive again.
yes, the purchase of a few hundred francs of ETFs cost around $0.3, which is virtually nothing. the only reason to limit the number of ETFs is to reduce complexity, which is a reasonable wish. Out of experience I can tell that (passively) managing my 6-ETF portfolio is really simple 