In my opinion the message should be as simple as possible.
Recommending software is difficult, as each user has its own needs, and what is good for one person might be incompatible with other one OS (for example, there is no electrum for iphone).

On the end, what people need is knowledge/education about bitcoin and blockchain. However, most people who leave funds on exchanges are just trading and want a quick buck.

Education is an act that requires effort. It is not possible to educate people with a message, even if it is a complete and long one. A short message can spark curiosity and make many people search about private keys and solve their problem.