Alternatives for expression "is that"

There are certainly other expressions though you may need to alter the sentence: e.g. entails

to cause or involve by necessity or as a consequence: a loss entailing no regret.

They key benefit of using A entails the possibility of you making [or: the potential for you to make] more money.

Synonyms: involves (although somewhat weaker as it would only be part of the key benefit in your example) or simply is (... is the potential for you to make more money)

For your second sentence, depending on the precise context you might just write:

But [or however or unfortunately] you have to do it by yourself.

An alternative might be to add a clause onto a previous sentence:..., the drawback being you have to do it yourself.