[One (M9) Q&A, Help & Troubleshooting] whereitsat42:
Gonna try my best to give as much information as I can to make it easier for anyone good enough to try and help me with this. So I have an HTC One M9, from Sprint right here in the good old US of A, and it was working fine until I got the OTA Marshmallow update. At that point, apps started crashing like crazy (even after all were updated) and the status bar at the top of the screen and navigation buttons at the bottom would randomly disappear for several seconds after pressing the volume buttons, even after wiping the cache for both the system as well as the individual apps in question.

"No problem," I thought, "I'll just root it and install a custom ROM like I have with all my Android phones in the past." So I did that and tried Android Revolution, ICE, and ViperOne (all the latest versions), and every one of them had the same issues with programs crashing, the volume buttons messing up the screen, and so on. Actually, I couldn't even get through the setup wizards for any of them because those kept crashing too, and I had to use TWRP to rename the HTCSetupWizard.apk just to get to the home screen. Once there, my home button was inoperable, and no notifications or usual buttons like WiFi, Bluetooth, etc appeared when I pulled down on the status bar (which I discovered is what Android does if you don't complete the setup wizard).

At this point, I thought, "Well, maybe it's just a problem where my phone doesn't like Marshmallow for some reason. Let me try downgrading to Lollipop and see what happens." So I got the latest Lollipop versions for each of the aforementioned Custom ROMs, and they all ran smooth as silk, which tells me it's not an issue with faulty or damaged hardware. Only problem was that mobile data wouldn't work, on either LTE or CDMA settings.

I tried changing network modes, popping my SIM card in and out and rebooting in various combinations, but none of it worked. I couldn't even get into APN settings since the phone couldn't seem to detect my SIM card and had the APN menu greyed out. I thought I might have somehow damaged the SIM card with all the time I spent screwing around with the phone, but just to see what happened, I flashed a Marshmallow ROM back onto the phone and, sure enough, LTE service was right back.

I did some more research and saw that the issue may lay in the fact that the firmware (which is currently OS version 3.41.651.4) is not usable for Lollipop OS versions, and since my phone is S-ON (and I'm really not interested in spending $25 to S-OFF it), I'm not able to downgrade the firmware or, I assume, the radio firmware (if that's something you can even do separately from the "regular" firmware. Currently version 3.00.C1144084@60114).

At this point, I'm really not sure what to do, and could use some guidance. I feel like my options basically boil down to the following:

1)Find a way to make Marshmallow ROMs more stable, possibly by reinstalling the firmware or some other method someone here might know that I don't,

2)S-OFF my phone, downgrade my firmware to something Lollipop can work with, and try a Lollipop-era ROM (not ideal), or

3)Go back to stock and hope Sprint pushes down an update my phone can use.

After going through all that, does anyone reading this have a recommendation for which of these options would be best (and how to go about them), or possibly offer other ideas I haven't come up with during the week and a half I've been researching and trying to fix it myself? I'm not hellbent on any particular option, I just want my phone to work (all of it) without constant crashes and glitches. Thank you to anyone who read all that and is still moved to offer help! :)