The energy unfriendliness of BTC is what supports it not having to be under pinned by anything else, i.e. for proof of work to be hard and doable by a computer

The energy unfriendliness of BTC is what supports it not having to be under pinned by anything else, i.e. for proof of work to be hard and doable by a computer it must be energy intense. If underpinning by a legal authority is acceptable to you then many proof of stake cryptocurrencies would likely be appropriate.