@craigbot

the only way I can interact with the GUI is the cursor moves super fast and uncontrollably (for me) with the left analog switch, and no button that I press selects anything. So a few questions related to that:
Can an Xbox controller be set up to work easily with Amiberry? If so, how?

I'd imagine so -- it works with other gamepads -- but I do not have one to test with.

I can't replicate this on my (Logitech) controller; Dpad and L-analog both move the highlight cursor one list-item at a time, and holding in a direction does not scroll. Pressing the A or B buttons ("south" or "east") , selects the current item.

When you say "cursor moves super fast" do you mean it's scrolling the highlight through list-items, or is it like free-moving the mouse-pointer?

Have you done any other edit/customize controls besides setup the Xbox gamepad in EmulationStation?

If not, should I be using a keyboard and mouse plugged into the Pi4? If so, how can I have both an Xbox controller for the RetroPie in general and a keyboard and mouse for the Amiga?

Keyboard and mouse should work fine, and might be required for some games. There is no problem having them all plugged in at the same time, they don't interfere; Pi have four USB ports which should be enough for a kb, mouse, and 2x gamepads. You can use a hub or splitter if you need more ports. I keep a kb plugged in at all times and plug the mouse in or out when I want to play some Starcraft.

The second issue: when I bring up the game, I get this screen:

Looks similar but not quite the same as when I had the wrong filename on my kickstarts. Can you try run and post the output of

ls -l /opt/retropie/configs/amiga/amiberry/whdboot/save-data/Kickstarts/kick34005.A500

And also whatever file that path points to if it's a symlink:

ls -l "/path/to/that file.xyz"

If I click on Start Amiberry, I get this screen (...) will I need to configure this, or is it pretty much set up out of the box?

You shouldn't have to do anything here, or at least I didn't. This is just what happens if you start the emulator interface without loading any particular rom.

WHD autoboot /data