To avoid volatility of bitcoin, you can convert your cash into stablecoins like USDT, USDC, or BUSD which has the same value as the US dollar then store it in a non-custodial wallet or hardware wallet. Just keep the private key in a safe place that you only know, with that you already have your own bank. Just prepare a small amount of altcoin using those altcoins, I'm using trustwallet as my savings account for every transaction you make iit needs a small amount of Tron(TRX) when using USDT while BUSD uses BNB in smart chain wallet. As for USDC, I never tried having that stablecoin because only few wallets support it so I don't know what coin does it uses for fees.