My adviser keeps changing his mind about my papers. What should I do?

I experienced a bit of what you're describing near the end of my PhD. I got a lot of good advice from colleagues that had completed their doctorates. Here's how I dealt with it.

Remember that it's your PhD, and you are (mostly) in charge. At this point, you should have a pretty good idea of what's really required for a PhD. (If not, skim more dissertations from similar research projects.)

There will always be more that could be done to further the research. There's always another technique you could try, another analysis to do. Your goal should be to do what's required to finish the PhD, no more. (Of course, you want to do quality work, but if you've made it this far, I doubt you'd settle for less.) You can always continue the research as a postdoc, or let someone else continue it. That's what the "future directions" section is for in your thesis.

Your advisors are focussed on the research itself, and the logical next steps, rather than on wrapping up your PhD. That doesn't mean they're bad people, or deliberately trying to frustrate you. They're just human.

Sit down and figure out what you think needs to be done before you are ready to finish the experiments, finalise your thesis, and defend it. Then meet with your advisors and get their feedback. If they suggest doing more experiments or analysis, beyond what you think is necessary, you might say something like "I agree that's a good future direction, but I think that could be done post-PhD." Your goal is not to convince them that X doesn't need to be done, it's to discuss whether X needs to be done before the PhD can be awarded. Negotiate things from that point-of-view, and I think you'll find it easier to reach agreement.

One of my colleagues went through a similar problem when he was in graduate school: his primary advisor kept moving the goalposts on him as he was trying to finish all his papers and graduate.

There are no easy solutions here. The best thing I would do is use the fact that you have all of this work done waiting and ready to go out. It sounds like you already have enough material for a dissertation. Therefore, I would suggest that you compile everything you have done so far, and present it to your committee. Let them know that "this is what I have done that's waiting for peer review," and ask if everyone can come to the agreement that it's ready to go out. If not, then ask, as mhwombat suggests, exactly what needs to be done. Then stick to it. (It seems your advisor has a severe case of "perfectionitis"—good for him in the long term, bad for you in the here and now.)