migrating DBs from SQL server 2005 to SQL server 2012
Depending on the edition since the database size is big, you can look into 


Logshipping - set up with full backup and subsequent log backups. During the cutover, just take the final tail log backup and restore the 2012 server databases with RECOVERY. 
Mirroring - Set up as async and during cutover, change the mode to SYNC and failover. 


Once the failover is done, remove logshipping or mirroring.

I agree with alroc that the dbatools.io is extremely useful, but in your situation you are better off going with above and separately use the cmdlets for migrating jobs, logins.

For more detailed steps, refer to my answer here.