Grant, I received your message and I'll just reply here. I have replaced two brake boosters with used ones and there were no issues afterward. Both of the ones I replaced still held a good vacuum, but had internal problems. One had corrosion causing it to bind. Another had a leaking seal allowing brake fluid into the booster.
Your troubleshooting process looks logical, but before taking the trouble to replace it there are a couple more steps you can take to be sure. Check this page:
https://m.roadkillcustoms.com/test-power-brake-booster/.
You might as well do a full brake system flush while you're at it.
