Apple Mail won't stop Copying Messages for days following Monterey upgrade

You stated you booted from an external disk, if that disk is an HDD and not an SSD it will be significantly slower. Are you running Mac OS X Sierra on the internal disk? Dual booting from the external disk for Monterey? Monterey is a bit slower on 2015 MacBook Pros but it sounds more like booting off an external HDD is slowing you down considerably. Yes, running Monterey will demand more of your hardware and can run slower than older operating system versions. It will install and it will run but it may not run nearly as fast as newer hardware. However, it should run rather well even on a 2015 Mac.

There is one Major Concern, do you have the Apple original internal SSD or did you upgrade to a 3rd party SSD? There have been significant issues with Monterey installing on 3rd party SSD's requiring the original Apple SSD in order to install necessary firmware updates so Monterey can fail to upgrade in those scenarios. If you can install the original Apple SSD internal disk, then upgrade to Monterey then swap the drive back to the 3rd party one and upgrade again, that seems to work for those impacted by this complication.

DO NOT UPGRADE TO MONTEREY DIRECTLY FROM SIERRA! That may result in catastrophic problems. That's several (five) operating systems old and making a leap skipping even one version of macOS is dangerous. You should make a full backup with Time Machine of the Sierra system and a full clone backup using SuperDuper or CCC - Carbon Copy Cloner to an external drive. Then proceed to upgrade from Sierra to High Sierra to Mojave to Catalina to Big Sur before attempting to upgrade to Monterey. You can obtain the installers for older macOS versions here: How to get old versions of macOS - Apple Support You should upgrade slowly, ensuring everything works after each upgrade, then making new backups before upgrading to the next subsequent release. If you run into problems you can restore a backup to the previous release. You should buy a few 16GB+ flash drives. Burn the current operating system version to one and the new operating system version to another. This will give you the option of booting easily into the prior release to wipe out and reinstall the previous macOS so you can restore your latest backup. Obviously, this will take time but you haven't been upgrading for 6 years. There have been massive changes to macOS that will break stuff if you make to large a leap, skipping operating system versions. Apple doesn't test upgrading from releases that old. 3rd party software such as security software will definitely cause problems. You should uninstall all of that stuff. Anti-virus, anti-malware, cleaner Apps, etc.

Many software developers and operating system vendors are starting to consider any software older than N-2 (current minus two years) to be beyond support. That means many macOS App developers won't support any macOS version that is more than 2 years old. If you do not keep up-to-date within 2 years of the current OS version not only will Apps cease to be updated or even available but you are not receiving operating system security fixes and that makes your computer vulnerable to being hacked.

Your Mac is 7 years old and that is right about the time when you'll likely have it dropped from support by Apple this fall when the next macOS version after Monterey ships. The 2013 & 2014 MacBook Pros are considered "Vintage" and the 2012 is "Obsolete". Your Mac will likely be considered "Vintage" by Fall 2022. It is time to consider a new Mac. You have perhaps one or two years before you start running into software problems because you won't be able to run the latest macOS. Not to mention you might experience battery problems as the age of the battery is considerable unless you've had it replaced.

The new Apple Silicon M1 processors are running extremely fast compared to the Intel Macs. By next year there won't be any Intel Macs still being sold.

Running Windows on an Apple Silicon M1 Mac will be problematic. The M1 processors are ARM64 processors and are not X86_64 compatible. So any software you need to keep running will need to be Universal binaries compiled with both ARM64 and x86_64 Intel. Also all software must be 64bit as well, 32bit will no longer work. Otherwise macOS on M1 can use Rosetta 2 to covert the 64bit Intel binary to run on ARM64 but it doesn't work for everything, especially virtualized operating systems. You cannot run x86_64 Windows on an M1 Mac. There are reports of people running Windows ARM with Parallels but that is entirely NOT Supported by Apple nor Microsoft. You could buy a cheap small PC like an Intel NUC and set it up then run it headless and use Microsoft Remote Desktop on your Mac to remotely connect to it to run anything you cannot run on the Mac.

Quicken isn't the only game in town but change will be necessary to adapt to another accounting platform. For basic accounting in the personal sense there are several cloud based online solutions. For small business there are other solutions as opposed to QuickBooks. But again, no competitor is going to be identical to the way Intuit does things.

The new Apple M1 Macs are outstanding, they run very cool and wicked fast. Mind blowing performance, massive battery life and instant on, etc. A considerable improvement all around. It will be like night vs day. The internal SDD's run about 10-15 times faster than what you have now. The RAM is many times faster. The processors are lightning quick. Even the entry level M1 released last year is impressive. The M1 Pro is better and the M1 Max / Ultra are really intended for heavy processing needs that most people really do not need. I would recommend a minimum of 512GB of storage and 16GB of RAM on any new purchase to account for future needs.