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.
However, looking at the crypto market I have not really been able to find a coin like that. Have you got any ideas/suggestions?
You mainly have 2 options, lending or staking.
This is a good site for tracking the interest rates you can earn by lending:
https://defirate.com/lend/Keep in mind the risks. Defi platforms are new and relatively untested, so there could be potential unforeseen losses from bugs, network congestion, liquidation problems, etc. Centralized platforms carry the obvious caveat, "not your keys, not your coins."
Staking refers to proof-of-stake altcoins. You can currently net over 9% APR by staking ETH:
https://launchpad.ethereum.org/