slow startup - iMac from late 2015 running latest Monterey

If performance drastically improves when using a new user account, then Login Items from the main user account would be the first & easiest thing to deal with since you can easily disable the Login Items to see if that makes a difference in the main user account. However, you did not notice much difference while booted into Safe Mode which also disables Login Items in addition to most of the other items listed in the EtreCheck report. Surprisingly, I have experienced a few Macs in my organization where Safe Mode negatively impacted system performance....I don't know why as it doesn't make any sense.

You should definitely remove DropBox from the Login Items of your main user account to see if it makes a difference. I have learned from another respected more knowledgeable forum contributor that most third party file syncing services cause performance issues because of how these apps access the file system (includes Drop Box, One Drive, Google Drive, etc.).

So that may indicate a configuration issue with the main user account, perhaps a corrupted preference file. No idea how to determine which preference file could be corrupted. Have you tried to close all open apps in your main user account (use the File --> Quit option, not the red close button). Then when rebooting the Mac, make sure to uncheck "Reopen windows when logging in" for good measure to see if this makes a difference. Relaunching apps automatically each time does not allow them to clean up after themselves so this procedure may help.

I would be concerned whether the hard drive is beginning to wear out or even fail. It is possible the area occupied by the main user account on the drive is on a bad sector which could also explain why the other user account gives a performance boost. You can check the health of the SSD and hard drive by running DriveDx and posting the complete text report for both drives here using the "Additional Text" icon which looks like a piece of paper.

You may also want to run Disk Utility First Aid on the Fusion Drive. If the Fusion Drive is not shown on the left pane of Disk Utility, then click "View" and select "Show All Devices" so that the Fusion Drive item appears on the left pane of Disk Utility. Even if First Aid says everything is "Ok", click "Show Details" and scroll back through the report to see if any unfixed errors are listed. If there are any unfixed errors listed, then those errors may be the source of the problem. You can try running First Aid while booted from Recovery Mode, otherwise it will require a clean install of macOS to fix which involves erasing the Fusion Drive before reinstalling macOS & restoring from a backup.

Except for the cloud syncing services, I don't see anything really concerning with the apps listed in the EtreCheck report, but I am not a software expert. I do know that Adobe Flash is no longer supported so you should uninstall any Flash software such as PepperFlash.

What are you saving to the Dropbox? If you have your Photos library there that's a big NO-NO. It must be on a volume that's formatted APFS or OS X Extended (journaled). It's been known to interfere with systems performance.

The Fusion drive may be failing: Its read/write speeds seem lower than they should be:

Write speed: 344 MB/s

Read speed: 850 MB/s

You might consider getting an external SSD, cloning your boot drive to it and boot and run from the SSD. It could be at least 4-5 times faster.

And you seem to have PACE anti virus software installed or parts of it left over. First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Uninstall PACE according to the developer's instructions.