As well as the tour which you have booked, Frankfurt Airport has an official visitors terrace which you can access from Terminal 2:
https://www.frankfurt-airport.com/en...isitorsterrace
You can also take a flight in a simulator from ProFlight which are from Lufthansa Aviation Training
https://www.proflight.com/en/standorte/frankfurt.php
In terms of other spotting, this guide is helpful:
http://www.frankfurt-aviation-friend...g-locations-2/
Spotting position 1/2 is accessible via public transport from the S-bahn station Zeppelinheim. For the rest it is a long walk or probably a car/taxi ride. Or you could maybe rent a bike to reach the locations.
But spotting position 1/2 is actually very good if they are landing from the west (on runways 25)