Beachballs in Monterey

There are several possibilities for the poor performance:

(1) Hardware issue. Bad memory chip, for instance. Can be addressed with Apple Hardware Test or Apple Authorized Service Provider (they have better test tools). I think this is unlikely, as you have a 2019 Mac.

(2) Problem with the external SSD or connection to it. Connect it directly, don't go through a dock. Are you using an adaptor? I think this is also unlikely to be the problem.

(3) Something brought over from the old system when you "cloned." I think this is the most likely. Cleanest way to get past this is to do a fresh/erase/clean install on the external and then migrate only user accounts and files, nothing else, on first boot, which will be like a new computer at that point, where you use Migration Assistant to migrate from a backup just user accounts and files, nothing else. You will then have no extensions or third party software. You can install the third party software again, be careful to avoid all VPN, security, anti-virus, "cleaner tools," etc.

This post has external drive system install instructions, courtesy of Jack19.

Use an external SSD as your startup disk … - Apple Community

Be sure that your computer fits the requirements, I did not see it in the list in that post.

Random reboots can be caused by hardware problems. Since your old internal fusion drive is still mounting and running, consider checking all its components, first with Disk Utility, then with DriveDX. DriveDX is good for identifying disks that are failing.

If the internal drive is healthy we would have to look elsewhere. However if it is failing, a connected failing or failed drive can hang a Mac system For instance, connecting an external drive (I have done this) that has failed can completely freeze a system. Then when the bad drive is disconnected, everything is normal again. This is pure speculation, but it is easy to check with DriveDX and Disk Utility.

You can also run the Apple Hardware test or take the Mac to an Apple Authorized Service Provider where they have more comprehensive tests for all the hardware elements.

Run DriveDX Full Self-test, it may take an hour or more. Run it on the fusion drive or on each piece of the fusion drive (I'm not sure whether DriveDx does each piece separately or together).

What do the health indicators show for the HDD and SSD pieces of the fusion drive? There are many such health indicators that DriveDX provides, basically all the SMART parameters and recorded diagnostics. If possible, save the report and post it in a reply here.

I'm not even sure how to interpret it if the report indicates a degraded internal fusion drive because you are running from an external SSD. I'm just not sure whether something with the fusion drive that remains connected but is no longer being used could be causing this.

I am suspecting that something could also have brought over from your old installation.

Did your new setup with the booting from an external SSD ever work properly?

2019 is not an old Mac by any means. Also, I suspect the association with Monterey is a coincidence, Apple sells millions of Macs with Monterey. My own experience is that we have three 2019 Macs running Monterey with no issues, one 2015 Mac running Monterey with no issues. In fact my employer has thousands of Macs in use by employees all running Monterey.

Let's see what the diagnostics reveal about the fusion drive, and maybe someone will weigh in here with something to say about whether a problematic internal fusion drive can cause kernel panics when the boot drive is an external SSD.

Are you unable to run the extended DriveDX test? In any case, what is shown in your report seems to indicate that the fusion drive is ok. So I doubt some issue with it is hanging your Mac up.

You indicated that you brought over all your old system via a clone copy to the external SSD. I see remnants of security tools still in your Etrecheck report (tunnelbear) and you mentioned earlier that you removed other software using "find my file" (were you using Find Any File or the Mac's built in Find command?).

What is really needed is to run complete uninstaller, as some files remain hidden and have been found to still impact the operation of the Mac unless completely uninstalled.

You might consider doing an "erase/format and clean install" into your external SSD after making two verified good backups of everything. Then migrate to the clean installation only files and folders, no settings or applications or anything else. Then install everything fresh into the new system. I have done this and it is not that time consuming. I am suspecting that you brought some residual issue over from your old install and it is slowing things down. There are many crashes and hangs still reported in your Etrecheck, so you might benefit from the erase and clean install described above.