A raspberry pi is extremely overkill for such a task. You could easily do this with an E-Ink display connected to an ESP32 or maybe an ESP8266. These have about 4mb of memory (depending on what board you buy) and onboard wifi/bluetooth. They cost about $5.
I’ve made a tool that can convert your pfsense configuration to an opnsense configuration. It’s not perfect, but it can do a majority of the legwork for you.
You can pull the site via docker:
docker run --name pf2opn -p 4200:80 -d mwood77/pf2opn
Or use it on the web here:
As always, please try the converted configuration in a test image before you apply it to a production environment. If you find any bugs, please reach out.