For example, how would one translate "this is harder than you think"? My initial thought would be something like 这个比你想更难, but I don't know whether that sounds idiomatic in Chinese
I would translate "It is harder than I/you think" as 似易實難.
As for "....than you think", the literal translation is "比(我)想像中的更....". I don't think there is a more idiomatic way to say it.
这比(我/你)想像中的更早 = earlier than I/ you think
这比(我/你)想像中的更難 = more difficult than I/ you think
这比(我/你)想像中的更差 = worse than I/ you think
pronouns in ( ) can be omitted
Well, you are getting there.
A sensible alternative is 比你想像的更难
1st, 这个 is too indefinite. It leaves the sentence "hanging" You'll have to say, for example, 这个任务, (this assignment), 这种工作,（this kind of work), etc.
Coming to 想. Well, just "thinking" alone by itself is "incomplete" because even in English you have to use, "imagine", "visualize", "consider", "picture", "envisage", "ponder" to cater for various kinds of "thinking"
Thus in Chinese, in the context of your sentence, you could use 想像, "imagine", "visualize" as one common option in this kind of situation.
Finally, you need to add 的 before 更难
There are many answers in this Forum on the use of 的. So, do a search.
Reading your question again, you mean to reduce the sentence to the "size", "to function" like a Chinese Idiom?
In that case, using your own attempt, you could contract it to 比想更难, (Harder than thought), because the other words like 这,个,你, are functionally superfluous.