Nitro 5 AN515-54-599H "Undoing Changes" boot loop, can't reach BIOS, WinRE, etc.

IanSM

I have an AN515-54-599H (added HDD in second bay and replaced stock 8GB RAM with 32GB soon after purchase in December 2020) which was working great until Saturday. It asked me to schedule a restart for a Windows update so I chose 3:30 AM as I usually do. Upon waking up Sunday, the following began to happen:

1. The machine turns on and the Acer logo appears for a few seconds, then a spinning circle appears below the logo for a few more seconds. (Note: If I do two hard shutdowns in a row, then in step 1 the Acer logo appears, then before any spinning circle shows up, everything turns off for a few seconds including the keyboard backlight, then turns back on and begins the loop at step 1, from which point it proceeds without doing what I have described between these parentheses)
2. The Acer logo and spinning circle disappear, and a command window flashes without visible content for just a few milliseconds. It may be shown against a grayed-out version of my Windows desktop background, but it's hard to tell for sure.
3. A black screen reappears with a spinning circle, and the text "Attempting to recover installation…" briefly appears on the screen for a fraction of a second (I may be missing a word for that reason). The text then changes to "Undoing changes made to your computer…" and persists for anywhere from a dozen seconds to a couple minutes.
4. The screen goes black but backlit, then the screen backlight and the fans turn off. The keyboard backlight remains on, and we return to item 1.

I am unable to enter the BIOS or any boot menu.

1. On the Acer logo screen I have tried tapping F2, Fn+ F2, F8, Fn + F8, F10, Alt + F10, Alt + Fn + F10, Fn + Alt + F10, F12, Fn + F12, Esc, Delete, Shift, and holding Shift, plus a few other similar things.
2. I have done three hard shutdowns in a row both on the Acer logo screen and in the subsequent screens, and the machine will not boot into WinRE.
3. I have created a Win11 bootable recovery USB from Microsoft's "MediaCreationTool_Win11_23H2.exe" hoping it would force a boot from the USB drive, but no dice, and no way to get it to boot from there.
4. I don't own any other PC to try yanking the SSD and troubleshooting from the other PC; my employer owns the PC I am using to ask this question.

Is there anything else I should try before I give up and haul it into a shop? Please don't suggest anything I've already tried.

IanSM

I guess I wasn't tapping F2 fast enough. Strange, 4 taps per second didn't do it but 5 did.

IanSM

Additional context: My main SSD was getting pretty close to full, though I do like to leave several gigs as a buffer and had done so the last I had checked, but I've heard of Windows updates causing things like this when the OS drive is getting close to full — if that helps figure this out.

IanSM

Also, this machine was running Windows 11, not 10.

IanSM

I guess I wasn't tapping F2 fast enough. Strange, 4 taps per second didn't do it but 5 did.