What is the best way to write a smart contract that will work across both Cardano and Ethereum? It doesn't have to be released yet but preferably something that will be usable by the end of the year. I believe that this is what the KEVM may allow?
This won't be possible, because Solidy and Plutus are both different languages and even the blockchain accounting system is different on both platforms.
There might be a side chain running KEVM in the future, where you will be able to run Ethereum smart contact and somehow interact with the Cardano mainnet, but this is still just in the planning at the moment and nothing specific has been shared to mention we will have this anytime soon.