Thank you so much! This worked for me. After entering my recovery key, I was able to access the command line in Recovery Mode. Not sure why this didn’t work before.
I dug into the user manual for my machine and it specifically warns against trying to run in ACHI mode. I wish Ubuntu was more upfront in the public-facing documentation about this being a requirement – I did not realize I would need to change the SATA mode until after already investing a lot of time into this, and would have chosen a different linux distro and/or computer model if I knew about this up front.
Will update if I do get the dual boot to work.