Thanks for replying, everyone! This project is temporarily on hold; downloading and sha256sum-ming some different isos led me to discover that my main system has a RAM issue. 🤦
I just learned to not trust motherboards’ automatic RAM voltage setting; only one of four sticks seems to be sensitive to the apparent undervoltage that it was running on.
Edit: I hope that whoever designed the style of RAM slot that has a latch on only one side stubs their toe every day for the rest of their life.