>I try to be nice but people will take advantage even if they don't think they are.
>They will say that they are getting the work done so those 5 hour days are justified. I don't always agree and as a working manager I'm working 10-12 hour days cleaning up the rest and need more help and that attitude doesn't help me.
Keep in mind that for about the first quarter-century of our lives, we are trained in everything that we do that someone else will tell us what to do and set the expectations, and if we meet them then we're doing good, if we exceed them then we're doing better and deserve a reward. First our parents, then our teachers, then our entry-level bosses. They know what needs to be done and how long it should take; if we finish early, they tell us what to do next (or that we've done a good job and can relax).
It's only later, if we happen to get into a creative career, that our bosses can only give us a vague idea of what needs to be done and it's up to us to work out the details, set expectations, and seek out other work to do if we finish early. That goes against the grain of everything we've ever been taught.
We also have to unlearn the bad habits that hourly wage jobs train into us. If you want more free time, get your work done quicker (or that if you work efficiently, you get paid less).
It may be that you are working with people who are still trying their best at doing exactly what they've been taught their entire lives, and expecting what they've always been taught to expect, not yet consciously realizing that the dynamics are completely different and 20+ years of trained behavior and expectations should be thrown out the window.
>If you finish early, there is usually nothing stopping you from helping out with other miscellaneous things than just the big projects.
Sure, if somebody's falling behind, help them out if you can. Or go for the hidden victories. Maybe devs are spending time every week dealing with the results of a recurring bug - find a way to track down the source so they won't have to spend that time every week. Maybe somebody in marketing is tediously copy-pasting things that could easily be scripted so they could spend more time on creative stuff. Or the business people could really use some data that they never even knew you could easily pull for them. Ask the IT team about their frustrations and you might find something that could easily be changed on the dev side to save them hassle and reduce friction. Time spent helping your coworkers is time well spent in more ways than one. Unfortunately, we're trained from birth to be competitive and passive rather than cooperative and active about things like that.
If possible, try to encourage them to understand that extra time gives them the power to be creative and really excel, possibly in ways that you haven't even thought of. If you must give them tedious or unpleasant tasks, try making it an opportunity for them to find a way to remove the tedium (or even the need for the task) in the future. If they still aren't interested, then maybe it is a difference of opinion on work/life balance (they're trying to maximize their free time while also doing a good enough job).