Just saw this. I'm a physics post-bacc at a national lab. Definitely worth it. I made another post about it under the thread about "What if I don't get in anywhere?" or something like that.
Let's say I never took any time off and had tried applying my last year of school (which I didn't do): maybe, maaaayyybe, I would've gotten into WVU. Maybe. Not that it's a bad school - it has excellent plasma physics - but that would be the best I could do. I know most of you are looking at really top-notch places (mine were scattered - victory post forthcoming after I hear back from everywhere).
I took the time off and did the work, and I'm really happy I did. There was also a period of time where I did something other than the post-bacc to just really consider my options.
Anyway, logistics: I don't know what the post-baccs at Ivy Leagues pay, but where I'm at, we get paid 1.5 to 2 times the lowest a grad student at a university could get paid. So we get paid quite well. You can save a lot of money, and it really lets you have flexibility in choosing how many places to apply to, and how many times you want to retake one of those [expletive] tests.
Working at a national lab also gives you insight into one of the industries you could enter after a PhD, instead of remaining in academia for your time off. The upside to the Ivies (or being at a university) is that you might be able to make contacts at a school you want to attend and show them you're hard-working.
Finally, I'm someone who had a very low GPA and low test scores, but I knew I was passionate about STEM and wanted to continue doing it. Going to the national lab gave me great research experience and something else besides classes to tell these schools that I could do the work.
I think your plan to do a post-bacc, wherever you go, is a good one. Unless you can get a funded Master's, you don't want to accrue any graduate school debt. Nope. Your GPA is at least solid and not below a 3.5, and you can take time to study for the PGRE to boost your scores. (Kahn is a MUST. They cover literally everything you need to know about the test.) Also, if you do the Master's, I plead you, get the best [expletive] grades you can get. You want to show you can excel at grad classes. That's the main point of doing a Master's instead of a PhD first, and also, you want to do as much research as you can while you're at it.
The point of a post-bacc is to get a leg up on grad school. Period.
Something else: Say you do a post-bacc at national lab or Ivy. You apply that year and still don't get in anywhere suitable to your career choice. Check with your advisors on this, but maybe, you can then do a second post-bacc to boost even further. It's possible.
Finally, a selling point about one of my friends: He's a post master's here at the lab with me. This is his third round of applications and he didn't get into PhD's (or the ones he wanted) his first two rounds. He did a Master's meanwhile, then transitioned to here. Guess what? He got into his top choice this year - Berkeley - for his field of study.