It's not a risk-free to buy BTC and ETH. in fact, if doesn't even matter what kind of coin it is but the risk will always there. BTC and ETH are also volatile. The risk to lose your money is always there once bitcoin and ethereum go down to the below price that you have bought it. There's no risk-free investment in the crypto. The major crypto may also make you lose a lot of your money too.

Im feeling skeptical with your method. It seemed to me if you will not able to get an apartment even when bullish market gonna come. You need to take the more risk by picking low marketcap coin.
The less marketcap owned by the token and the more chance to growth as it will generate you more money compared with the coin that has owned very big market capitalization.

This is the key when you are willing to get your apartment in the future.