Thank you @jhughes. I finally got your solution to work, but for others, I'd like to add that I had to add these entries (see bottom of post) to the params.ini file. For me, I did not have permission to edit the params.ini file located here: C:\Program Files (x86)\PLSQL Developer 13\params.ini

but I found I could edit the file here: C:\Users\[myUID]\AppData\Roaming\PLSQL Developer 13\params.ini

...and that did the trick.

PATH=C:\Oracle\instantclient_21_6
TNS_ADMIN=C:\Oracle\instantclient_21_6\network\admin