I'm part of a massive death march project that really seems
unlikely to be finished in shorter than six months. The company
has announced a 1% bonus for everyone if this particular project
is completed by the end of the year.
Counting this week, there are nine weeks left in this calendar year. Six months' worth of work is approximately 960 work hours. Spread that over nine weeks, and you'd need to work 106 hours a week, or 15 hours a day (including weekends). Four extra hours a day isn't going to cut it.
Spoiler alert: the project won't get finished, there won't be any bonus, and all of your families' holidays will be ruined for nothing.
I would tell my manager that while I don't mind working a bit of overtime occasionally, the current request is far beyond what you consider reasonable and won't significantly increase your output over the time period. Do the best you can for the rest of the year but don't burn yourself out. This is the textbook definition of "poor planning on your part does not constitute an emergency on my part". Your boss isn't going to fire you before the end of the year if, as you said, 25-30% of the work relies on you because that would guarantee that the project fails. Spend the next two months finding a new position with a company that treats their employees better and jump ship as soon as you get a chance. It doesn't sound like there are many options for salvaging your current situation, or that it's worth salvaging at all.
You do actually have a lot of leverage here, if you have the intestinal fortitude to follow through. Your resignation would mean the project is clearly beyond impossible for the remaining team to finish regardless of overtime. Your boss can accept that you'll work the normal hours that were agreed upon, or you can walk and let your boss explain to upper management how he drove off a critical resource and caused a project to fail. A manager willing to work you that hard already doesn't care about you at all, so don't waste energy trying to keep them happy with you. There's a risk that they fire you after the deadline but since it's essentially impossible to meet the target at this point, there's still that risk even if you did work yourself to death. Hopefully, you'll have started your new job long before it gets to that point.
Also, you didn't mention where specifically you are but, in many places, PTO is considered part of your compensation. If your PTO expires at the end of the year and your employer suddenly tries to prevent you from using it without adequate warning, you may have grounds to sue them for denying you compensation. That particular part of the issue would likely be a better as a question for law.stackexchange.com.