Gremlin

I was introduced to the pack around 2 years ago, and since then I've been playing it consistently at least a few times per week since, and even with the pack not being fully completed yet, you are looking at around 400 hours per world worth of content if you're not set on speedrunning it.
Unlike other long-lasting packs, like GTNH for example, you are not limited by the amount of time a recipe takes to complete, and instead by the amount of resources that your factory can produce and process, meaning that you can always optimize and speed up your factory if you're willing to put in the effort.

That being said, this pack is very difficult, you are basically forced to setup passive production of just about everything you unlock in your progression (that being kind of the whole point of the pack), so don't expect this to be a cakewalk you can complete by batch crafting everything in an evening or two. If you can manage to built a proper factory however, you will have very little trouble with getting through the quest book.