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Instead, I would like to use crypto coins (not stablecoins) and earn interest on them, without having to worry about any fiat currency they may be pegged to. I am not really searching for a big growth in the currency itself (a new Bitcoin not really what I am looking for now), but instead something of relatively low volatility, to earn interest and on which I can rely that it's not going to underperform fiat currency as exchange.
There is no such cryptocurrency that mines away money while you sleep. 
Be careful of scams that promise you the same.Only methods of growing your money while giving it a low risk of loss, is to deposit the coins on some legit business that is allowing passive income for its investors. Be careful of HYIPs/MLM schemes - those are scam.
Probably the only legit one is investing in a casino bankroll, IMO. But that too the ROI is small and that you have to hand over the money to another company. Currently I dont know which casino is allowing this, previously I have used cryptogames but they have removed this now.
Well, crypto is rarely low volatility and ethereum moves like crazy, but it is still decent, it is basically all he asked for. Sure there is nothing in the world based on a company that I would trust, no company would be good about how "we put money in, it gives us 10% back yearly" type of deal, it is not possible and that is why I think it is quite obvious you assumed something like that, but when it is ingrained in the system like proof of staking that is something definitely possible.