So this is what I've tried doing; not sure if it's the correct timing or not:
From a dead battery, plug in and hold power + Vol down, it just goes into bootloop
During the bootloop, held Vol Down during the black screen in between the white HTC splashscreen; still bootloop (tried this at various times in between the splashscreen loops; there's about a 5 sec gap of black screen)
Tried the same above, but with Power + Vol Down
Tried Power + Vol Down during the white splashscreen, the back/home soft keys blink 4 - 6 times, and releasing the power button while keeping vol down held, and then reboots into the bootloop again
Not sure what to do at this point, besides contact HTC, unless you guys have any ideas/proper order to press the buttons?
Edit: plugged it into my computer, and about 3/4 of the way through the HTC Splashscreen, I hear the USB plugged in noise, then immediately the eject noise. For a second I can see "Android Phone" listed under Devices and Printers. Tried running fastboot reboot-bootloader and it didn't catch it. Tried running adb devices during the second it's recognized, but it didn't see anything. Tried in both USB 2.0 and 3.0 ports.
Also tried pressing Vol down during the time its recognized, but either my timing was off, or it didn't work.
HTC One M7
Unlocked | S-Off | ROM: NuSenseSIX 7.8
Locked | ROM: Hellkat 4.4.2 (build 20140116) | Kernel: BMS 3.4.75
Locked | S-Off | ROM: Infection 2.9.1 | Kernel: Stock | Radio: 1.22.10.0421r/1.22.10.0424r | HBOOT: Stock 2.25
HTC Incredible (S-Off / unlocked)