@JavierCairus Sounds like you're on the right track but the issue you mentioned "error: no suitable video mode found" sounds more like a grub or bios issue.
You could try overriding some grub settings as mentioned in this article: https://ubuntuforums.org/showthread.php?t=2409853.
Also even if you are in blind mode, I don't know if you really need to see the screen at that point, just try discovering the network address with synology finder.
Thanks for the kind words on the guide I provided, if you want to dump your partitions for the usb here I can see if there is any feedback i can give.