I'm changing the solution to this first suggestion of yours, Activate Open App from List. I too quickly drew an incorrect inference from my first experiment with it. Yes, typing a letter removes all list elements not containing that letter, but that is not all it does: it also selects one of the elements in a way that is very close to what I requested in my OP. E.g., with this list of open apps--
Arc
BusyCal
Discord
Finder
iTerm2
Neovide
Obsidian
Thunderbird
--typing the first letter of any of them selects it except that typing "o" selects Discord rather than Obsidian. I'm mystified by that exception. Regardless, I can type "ob" to select Obsidian.