Installed Monterey, still getting Mojave login screen

Skipping 2-3 or more macOS versions on an upgrade is very bad idea. Apple doesn't test upgrades in that manner. Nor do the beta testers. It might work but depending on the Mac model you will likely run into a variety of problems such as the one you experienced. Upgrading any operating system is a major operation involving multitudes of inter-dependent complex changes. Skipping operating systems introduces unexpected problems.

I've witnessed upgrades that skip release versions break machines completely, especially on the T2 equipped Macs and your 2018 is definitely a T2 equipped Mac. I just had one of my customers attempt upgrading from Mojave to Big Sur skipping Catalina entirely and initially she was stuck with an Apple logo and progress bar but before she called me, she installed Big Sur via Recovery Mode and it completed the Big Sur installation but her data was inaccessible because it created a new local admin account and her original user account no longer existed. Fortunately, the 800GB's of data was intact in /Users/<username> and there was a Previous System copy of her Apps and system level data. I had to swap her work Mac then manually mount the encrypted volume of her broken Mac with a recovery key over Thunderbolt Target Disk Mode and rsync her data to the new Mac which took an entire day plus shipping time.

If you have a full backup from before you upgraded to Monterey, I would highly recommend reverting back to Mojave, restoring the data then upgrade to Catalina and apply any and all patches, upgrade to Big Sur and apply any and all patches and then upgrade to Monterey. Yes, it is time consuming. but you need to receive all the firmware updates to the T2 BridgeOS properly. If executed in sequence, all the data and configurations should be carried over from macOS to macOS version.

The problem is that T2 equipped Intel MacBook Pro's 2018+ are factory encrypted. When you enable FileVault all it really does is generate a recovery key and add a private key to the Secure Enclave. It doesn't need to encrypt the disk because the disk is always encrypted. The issues you ran into are because your T2 chip running BridgeOS ran into problems. You missed a few firmware updates during your leap from Mojave to Monterey.

Alternatively, you could ensure you have a backup of all of your data. Then you could boot to Recovery Mode, open Disk Utility, erase the internal Macintosh HD and then re-install Monterey from scratch. Then restore your data from backup.

As for alternatives to Adobe Creative Cloud (depending on your specific needs and workflow) Affinity Designer, Photo, and Publisher could replace Illustrator, Photoshop, and InDesign for most people. They have a 30% off Black Friday sale at the moment. No subscription, just purchase new version when they hit a major release with free updates in between. Even full price their Apps are far less expensive than Adobe. It would be a bit of a learning curve as there are differences. There's Luminar by Skylum.com to replace Adobe Lightroom. For video editing, either Final Cut Pro or DaVinci Resolve instead of Adobe Premiere.

FYI, there are ways to download older macOS versions:

https://support.apple.com/en-us/HT211683

> Thoughts?

Backup, erase device, clean install, restore.

Well you should be able to boot from the CCC backup then use CCC to clone back to the internal disk. Hold the Option key while booting. Choose to boot from the CCC disk.