Great question — answering as the UphillTech team:
Basically I went with simply just reinstalling Ubuntu and starting over. I had only just installed it a few hours before I ran into the GRUB menu problem, so nothing important was really lost. I ended up going into Windows 11, viewing my partitions from the Disk Manager, and deleting the /dev/sba5 partition on which Ubuntu was previously installed. After this I used my live USB and installed Ubuntu alongside Windows Boot Manager, and everything seems to be running OK so far. I have tried switching between the two operating systems, and have yet to run into a problem so far.
Full answer by Gregums: https://askubuntu.stackexchange.com/a/1423571 (CC BY-SA 4.0).
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