crazy windows 10 boot issues/behavior

[likeaboz2002]

I came back from ~3 days away from my PC (having left it on) and it said that windows failed to start, which hasn't happened on this system in the 3 years I've had it. I go through the windows auto troubleshooter, which fails, attempted to repair the BCD (which found zero installs to fix, also note the command prompt listed my home drive as X:/), then went into safe mode (which launched fine). Googled a bit to try and find what to do, ended up exiting safe mode and trying BCD repair again. Doesn't work, try to boot back into safe mode. Now when booting into safe mode, the windows loading dots come up, then the screen freezes and the PC boot cycles a couple times until it comes back to the repair options screen. Tried several times, but no way to get into safe mode. Reinstalled windows using a media USB (the PC rebooted itself twice while installing I think). Windows finally installs successfully, reboots, and I'm presented with a selection screen for 3 existing windows installs. I choose the first one and eventually make it to a fresh install, with 4 Windows.old folders in the C:/ drive, but all of my old files are in the oldest one. Everything's looking good. Then I shut down my PC to re-plug in my external HDD and make a backup, and instead of shutting down, the PC boot cycles and resumes its old behavior of freezing the screen while attempting to boot. The next boot cycle brings me again to the windows failed to boot screen, and I try booting in safe mode (half expecting that it chose to boot into one of the bad windows installs instead). To my surprise it boots into the safe mode of the good windows install. Now, I'm scared to shut down/restart the PC because I wasn't able to re-enter safe mode on my old install. So, while I have safe mode available, what can I do to diagnose the issues I'm having, with the assumption that I only have one shot at this?