Use Report Server Configuration Manager on source server, click on Encryption keys and backup. You’ll need these on the destination server

use Report Server Configuration Manager  and stop the service on both servers

detach ReportServer and ReportServerTempDb on both servers

I advise on the Destination Server making a backup of these two database

Copy the ReportServer and ReportServerTempDb  and the encryption key file to the new server and attach them in Sql Server management studio

Start the service

Click on Encryption Keys, restore and restore the key file generated on the source  server.

Subscription users are going to be different, you have to update the users to the new server. They’re in the format of ServerName\username

select * from subscriptions  in ReportServer database.  Run a simple update query.

Done