Erase and reinstall Monterey

For a migration from El Capitan (which I have done in fact), it works best to start fresh from the "vanilla" new Mac with nothing on it, and then migrate over, on the first boot up before there are even any accounts, just user accounts and files, no settings, no applications, no "other." Then reinstall software anew from scratch, and only from the latest versions of the installers, and only software you need. It also helps to check that the software you are bringing over is compatible with Monterey.

This article explains how to restore your Mac to the "vanilla" state it was in when new.

What to do before you sell, give away, or trade in your Mac - Apple Support

Don't drag applications over or migrate them. This won't work well.

Assuming you still have your El Capitan Time Machine backup, you can still use that to migrate from. If you have updated that backup after your set up the new machine, it probably still can be used, but again, just migrate accounts and files only, for the smoothest transition.

If the new computer is going to be the Apple Silicon M1 / M2 Equipped Computer - would suggest Only Migrating the User Account and nothing more.

Some have had success and others have been left with a mess when Migrating everything.

1 - Apple Silicon with either Big Sur macOS 11 or Monterey macOS 12 Only Support everything 64 bit and there is no workaround.

2 - Issues that may exist on the older computer will be migrated to the New Computer. This would badly pollute the new Original and Pristine installation of the new OS.

3 -  Applications, extensions and Software Drivers may or may not even work on the new computer.

4 - IMHO - it would be best to only install new application etc Directly from the Developer or the Apple Apps Store.

5 - Due Diligence on New Applications and make sure they are Monterey / Big Sur compatible and factor in the ARMs CPU of the Apple Silicon Computer  

6- To Start Over like " Almost Out of the Box Experience " refer to below image. 🇨🇦

When you run Setup Assistant (on first boot after restoring your Mac to factory settings) or Migration Assistant, if you choose to migrate ONLY user accounts and files (nothing else), your Mail, browser settings, Contacts, Notes, Skype contacts etc. will all migrate over to the new Mac.

Alternatively, you can use iCloud with your Apple ID (which is free if you don't already have one) to sync Contacts and Notes and Safari bookmarks through iCloud. You can also tell Skype to sync your contacts with your Mac address book. As for Mail, it is smoothest to use the Setup/Migration Assistant to bring Mail over. But if your Mail is GMAIL or IMAP, you can simply recreate the accounts on the new Mac and the emails will download.

P. Phillips explained why it is best to migrate over only the user account and files, nothing more.

Thank you for having been pro-active and suppling the Etrecheck Report up front.

Having looked at the report, in detail, see nothing unusual in the report that would warrant and Erase and Reinstall aside from

" These are working in a separate test user "

I will dissect it, in detail. 

Will get back to you shortly

I think @ steve626 has covered that specific question very well IMHO

Using the Method, in image above, will achieve the Erasure process and the reinstallation will naturally follow.

Transferring Mailboxes and contents from … - Apple Community

Similar but not the exact Same question

AmericanadianTO wrote:

Thank you, I didn't know about syncing Skype. I synced Brave today.

Can I use Setup Assistant to get mail from two different backup partitions? I lost some in a transfer crash at one point.

Sorry I haven't been keeping up with your queries. At work all day, not able to check Apple Discussions all day ... fell behind here. Anyway, unfortunately I don't know of a way to get Mail from two different backups.

However there are still some things you can try, might or might not work. On your Time Machine backups, try to explore the backup files using the Finder (instead of the Time Machine interface). Don't change anything in there, just use the Finder to browse the folders and files. Open the folders like you usually do. You will be looking for the files with your Apple Mail. I don't have an El Capitan Mac right now (you said the old Mac was under El Capitan, correct?) but currently, Apple Mail files are inside the User Home folder => Library => Mail folder. (The Library folder is normally not visible, see below for how to make it visible in a Time Machine Finder view.) Inside that Mail folder, look for folders with names like V5, V7 ... Pick the highest numbered one, check that it has some decent file size (it should if it has emails in it). Bring a copy of that V5 (or whatever it is numbered) folder over to your new Mac. Then on the new Mac, open the Apple Mail program, and use the Import function. Select that "V" folder and import the emails from that folder. A new Mailbox called "Import" gets created with the emails imported from that "V" folder. See if it has the emails you are missing. If your backup has other "V" numbered folders, you can try importing those as well. Easy to delete the imported emails if they don't have the missing emails you are looking for.

If you are having trouble viewing into the Library folder in your Time Machine backup read below:

To unhide and view the user Library in the Finder view of a particular TM backup:

Navigate to the backup on the TM disk you wish to view. Select your Home Folder in the user folder of that backup. Right click on your account name (control Click) and then hold down the Option Key. Select <copy "account name" as Pathname>. Then, go the the Finder, select Go from the menu, then Go To Folder and paste the copied pathname to the window. Add /Library to the end of the pathname in the window, then hit Go. A Finder window should open that Time Machine backup user Library folder. Inside that folder should be a Mail folder, and inside that Mail folder will be the V3, V5, V7 ... folders (I don't recall which V-number corresponds to El Capitan but you will want the highest numbered one).