A few screenshots of some example boot error screens (right before it reboots and cycles with the same error messages).
OCR of the text (apologize for any OCR errors, I’ve edited it briefly to fix up a few things, but thought it might help future searchers):
Scanning mmc 0:2...
Found /@boot/extlinux/extlinux.conf
Retrieving file: /@boot/extlinux/extlinux.conf
2938 bytes read in 27 ms (105.5 KiB/s)
1 NixOS Default
2 NixOS Configuration 4 (2022-02-24 21:34 22.05pre356135.7f9b6c2babf)
3 NixOS Configuration 3 (2022-02-25 03:59 22.05pre356435.7/9b6eZbabf)
4: NixOS Configuration 2 (2022-02-24 21:34 22.05pre356435.7/9b6c2babr)
5: NixOS
Enter choice: 1: NixOS - Default
Retrieving file: /@boot/extlinux/..nixos/zhbharzhzqga617uc37uk0g6q1bx891u-initrd-linux-5.10.101-initrd
20588420 bytes read in 1910 ms (10.3 MiB/s)
Retrieving file: /@boot/extlinux/..nixos/6c1m6j0k1gmjrmikOyyizariiry?Zlqr-linux-5.10.101-Image
50629120 bytes read in 6863 ms (7 MiB/s)
append: init-mix/store/pikl8ju4077iaix36n8mifzu6l6rcajz-nixos-system-nixpi-22.05p re356435-7/9b6eZbabf/init console=tty1 console=ttyAMAO console=ttyS0, 1115200 root= LABEL=-NIXOS_SD rootfstype=btrfs rootflags=subvol=@ loglevel=4
Retrieving file: /@boot/extlinux/../nixos/6c16j0k1gmjrmikOyyizari1ry72lqr-linux-5.10.101-dtbs/broadcon/bcn2837-rpi-3-b.dtb
14310 bytes read in 66 ms (210.9 KiB/s)
Moving Image from 0x80000 to 0x200000, end=32f0000
## Flattened Device Tree blob at 03700000
Booting using the fdt blob at 0x3700000
Using Device Tree in place at 0000000003700000 end 00000000037067e5
fdt_find_or_add_subnode: memory: FDT_ERR_BADSTRUCTURE
ERROR: arch-specific fdt fixup failed
- must RESET the board to recover.
FDT creation failed!
resetting ...
