What I don't understand though is once all bitcoins have been mined, doesn't that mean that transaction fees are then also required to be proportional to the bitcoin market cap and electricity usage value?
No, fee costs are based on supply and demand of block space. It's simple, miners just scoop the highest paying transactions into the next block. If there's congestion, people pay higher fees to expedite their transactions.
Yes but once there's no longer the financial insentive of bitcoin rewarded for mining, fees need to have the same insentive relative to bitcoin market cap? Otherwise double spending becomes the most profitable for miners surely.