[cesarb]

> i.e. Bitcoin's energy usage depends on the difficulty of mining a block which is not affected by the number of transactions in a block.

Only if the transactions do not have any fee. If they have a transaction fee, this indirectly increases the difficulty of mining future blocks (since the miners get more Bitcoin per block, they can use more energy per block before their expected net return is negative, and they are incentivized to do so through competition with other miners; the Bitcoin difficulty adjustment then notices the increase in hash rate and increases the difficulty).