With the help of a good friend who kindly gave me some tips and things to try, I have succeeded at getting rbfeeder to execute using a non-privileged user account ("radarb" in my example below). In addition to modifying the rbfeeder.service file, one will need to specify a log file path in /etc/rbfeeder.ini to which the new account has write privileges. After making these mods, do "sudo systemctl daemon-reload" followed by "sudo systemctl restart rbfeeder", and away you go.
Best regards,
Andreas / VA2WBT
(For reasons I don't understand, I'm unable to share my code (it's telling me to enable Javascript, which is already enabled), so am attempting to provide it via an attachment image.)