I have a league on MFL that does it that exact same way.
To have rolling waivers, in Setup -> Waiver Request Setup, you set "Waiver Request Sort Order" to "The system maintains a
season-long rolling waiver order, moving the franchise that picks to the bottom of the list."
You set the very first waiver's order in Setup -> Custom Waiver Order. So once you know what worst to first will be, set it in there once and then you're good to go the rest of the season. Do not change the custom waiver order after that or you'd basically overwrite the rolling stuff, just leave it like that until next season's start.
First Come First Serve do not trigger the rolling waiver order to change. Only queued up waivers do.