You have set yourself quite a work program.
If you have a Time Machine backup for the old iMac you will make your task much simpler. I presume you have one. This is what I would do. Restoring from Time Machine is not just data but all the settings, passwords, email accounts, music, documents and so on.
You have four user accounts on one Mac and you are getting three new Macs. I'm guessing the two Airs are for your children and you get the Pro to share with another family member. You can restore the backups in a way that will separate the four user accounts on one old Mac across to the three new Macs.
Set up each Air using the Time Machine backup. Migration Assistant will ask you which user accounts you wish to migrate to the new Mac. Restore only ONE of the user accounts to each air - different user accounts naturally. On the Pro choose the remaining two user accounts. Time Machine might pop up during each of these migrations and ask if you want to use the existing backups on the new Mac. You don't. You will need the backup for the other computers.
On each new Mac proceed to sign-in with the appropriate Apple IDs. Once they are up an running each sign-in will have the same access to iCloud services as they enjoyed before on the iMac. Your 2TB storage plan will work as it did before. Reassure yourself after each new Mac setup that it is working. I have a couple of Macs and it's identical on each of them.
What data do you want to delete? Be aware that iCloud is not an offsite data storage system. It is a syncing service. If you delete files from your iCloud drive, for example, they will also delete from each of your devices AND iCloud. If you want to permanently delete files then do it before you start setting up the new Macs and then do a final backup. It is this backup that will restore to your new Macs.
Then decommission the 2012 iMac. Apple details the final steps you need to take in the support document called What to do before you sell, give away, or trade in your Mac
