If possible move it out of the attic. Do you really want to climb up there every time there’s a problem. Also you could do with a decent battery backup.

Run copper for that distance you gain nothing by using fiber.

Set the uplink port with the desired vlan tags or set to all traffic if that setting is available. Then configure the rest of the ports with necessary vlan info.

This is the way.

Ripping out coax and replacing with cat6 will be time consuming and costly. + if that coax is stapled down good luck yanking it out.

You can configure a non existent proxy server on the pos systems and they will not be able to access the internet only local devices .