The final stage of creating a death card seems to be selecting a portrait. However, there is no way to do so. There are no prompts, nowhere to click, and no key presses seem to do anything. Are others experiencing this?

Same here. On mac mini M1 and having the same problem. Also, I have beaten the first boss but the game didn't progress after that, it just got stuck on the empty table, with the guy looking at me

Guys, if you're on M1 try running it using Rosetta. It worked for me.
Right click on the "Inscryption" app, "Get Info" (or something) and find a "run using Rosetta" checkbox, that's it :)