Apple Robert gave the answer you need.
CarbonCopyCloner:
www.bombich.com
CCC is FREE to download (and use for 30 days).
Creating the backup will cost you nothing.
Once you have the backup done, it can be mounted in the finder and accessed like any other external drive.
But it's AN EXACT COPY of the iMac internal drive. You can even boot from it.
Now, you can wipe the iMac drive and do a "fresh re-install of the OS".
Best way to do this is by way of "internet recovery":
Command-OPTION-R at boot.
You'll need your wifi password if you're connecting via wifi.
The internet utilities take a while to load -- be patient.
When you get to the utilities, open disk utility.
If there's a "view" menu, choose
"show ALL devices" (VERY IMPORTANT that you do this).
If there IS NOT a "view menu", just disregard and go to the next step.
On the left, select the top line that represents the physical drive.
Click on it and erase it.
For High Sierra and earlier, choose Mac OS extended with journaling enabled, GUID partition format.
For Mojave or later, choose APFS with GUID partition format.
When the erase is done, quit disk utility and open the OS installer.
Start clicking through and let the install progress.
The Mac may reboot one or more times and the screen may go dark for a few minutes, then continue.
When the OS install is done, you'll see the initial startup screen (choose your language).
Begin clicking through.
At the appropriate time, setup assistant will ask if you wish to migrate data from another drive. Connect the backup if you haven't already done so.
Aim setup assistant "at the backup" and give it a few minutes to "digest everything".
Now, you'll see a list of stuff to migrate over.
Choose what you want and "turn it loose".
When done, you should see the login screen as you did before.
FOR MOVING FILES TO THE MACBOOK PRO:
Again, you can mount a CCC cloned backup right in the finder, then copy files and folders as needed.
HOWEVER...
A step you should take to eliminate permissions problems:
a. Mount the backup in the finder
b. Click ONE TIME on the icon to select it
c. Bring up the "get info" box (command-i)
d. At the bottom of get info, click the lock and enter your password (the one you use for the MBP)
e. Put a checkmark in "ignore ownership on this volume" (sharing and permissions)
f. Now you can copy what you wish, and whatever you copy to the MacBook will "come under the ownership" of the account ON the MacBook...