Im in a similar position with my Kickr Snap and looking to get a stages to perform outdoor workouts. I know my Snap reads lower.
However RE the training: if you perform the FTP test using your trainer then the FTP, whilst maybe incorrect, will ensure the workout targets are correct for the required adaptations. I.E you’re still working at the correct % of your FTP.
If you want to ensure you’re outdoor workouts are set correctly then im guessing you need to use your stages and power match. Then perform a new ftp test to set the correct FTP for indoor and outdoor workouts.