How to say no to more work as a PhD student so I can graduate
I think your most realistic option is to have an honest discussion with your advisor about your goals and expectations. 

Tell them that you’d like to graduate by 20xx, and you think your work on A B C was fulfilling and interesting. You feel like you can really build a narrative around these projects that would culminate in a good thesis within the timeframe set above. Given your goal, you feel like starting a new project at this time will be detrimental to your progress.

Ideally your advisor would totally see it your way and you’ll graduate into the sunset. Realistically: advisors tend to be overzealous at times and can exert a lot of pressure over their students. A reasonable advisor would leave the choice to you; a clever one will make you believe that it’s a really good idea to start this new project (you don’t have enough material now, your CV will be amazing with the new project accomplished...). So a lot of this depends on the dynamics you have with your advisor.

I would not go over their head unless you’re willing and able to switch advisors. Its hard to get back to a good working relationship after something like that. 

I will try to establish my own independence and capacity to successfully graduate with a plan (by 20yy I plan to submit these results, draft by Jan 20zz and so on). 

Good luck!