These are the checksums of the exact bootloader.img (should be from .7z) I used to succeed:
SHA256 3A4EB2AEAAE5C5C26D93C5F776E64A7BE3F2770A854E3388D575AA47E3D7A892
MD5 762898628404DBAF1A310DA262F2ADFF
16GB old samsung evo+ microsd
1GB MBR FAT (not FAT32) partiton, rest is unallocated
only bootloader.img on that partition
I believe MBR/FAT/1GB/total size of the microsd card did not make any difference as I said above, because I was testing exactly this and it did not make any difference at the time.
My suggestion is to atleast try to repeat both commands multiple times untill it, possibly at last, succeeds.