Did you run First Aid on the disk you were booted from? That is First Aid in "Live" mode and that can produce erroneous (and harmless) warnings because the disk being checked is still changing if it is the boot disk. The best way to run First Aid is from a disk other than the boot drive. This can be done by booting into Recovery, for instance, and running Disk First Aid from there.
You may find this article helpful:
https://eshop.macsales.com/blog/44661-first-aid-verify-and-repair-hfs-apfs-drives-with-disk-utility/
I also have a late 2015 iMac and recently replaced the internal HDD with a new internal SSD to improve performance. Also, booting from an external drive can lead to complications depending on how the MacOS was installed on that external. The latest MacOS puts impediments in front of booting from external drives, although it is not impossible: in fact Carbon Copy Cloner (CCC) and SuperDuper, both of which specialize in creating external bootable clones, both warn that these external bootable drives are not a long term solution and are best used for emergency only. CCC recommends not booting from its external clones. Also, people have reported that these bootable external drives, under Monterey, do not do MacOS updates properly; SuperDuper and CCC suggest doing the update on the internal drive and then recloning it to the external to get an updated external boot drive. I don't know if something like this is causing your problems, I think such problems are more common with newer Macs than older Macs (the newer ones like the T2 chip and M1 and M2 Macs have security restrictions that hamper external booting) but am mentioning it as possibly relevant.
Jack19 has posted a short guide to external boot drives: Use an external SSD as your startup disk … - Apple Community
A 2015 iMac is past nominal end of life so any use you are getting from it is a bonus at this point. Hardware will start to wear out, kernel panics are often due to hardware. The hardware issues can be intermittent, a failing but still sometimes working memory chip, a weak solder joint that does not cause failure until the metal heats up with a little use, etc. However, the fact you can boot into Safe Mode seems to indicate that maybe something installed is incompatible. You should remove those three questionable extensions and try again: the fact that they never caused problems in the past does not mean that they are compatible with the new version of MacOS you just installed. And Safe Mode disables extensions so that's pointing the finger maybe towards something installed that is incompatible with 12.6.1. You have to eliminate these possibilities in order to figure out if it is software or hardware causing the panics.
Also, Safe Mode disables certain graphics hardware on some Macs, so it might enable the Mac to boot even if hardware is causing the panics.
At this point, you need to boot into Safe Mode and immediately make one, or better yet, two complete backups of all your files. You have a fragile situation with an external boot drive and panics and unstable behavior and warnings from Disk Utility. I would at least start thinking about a new Mac. I still have and happily use 2010, 2013, and 2015 Macs, along with new ones, but I am also realistic about these older Macs, they all will fail and it's a matter of when not if. I have a minimum of two backups for each of my Macs. For the Mac I use for everything important (one of the newer ones), I have two Time Machine backups, two "clone" backups, plus manually store some files in Dropbox (in the cloud). This may be overkill, but never have I heard about someone complaining about having too many backups, but MANY times have I heard about loss of data due to not having backed up enough. These external drives used for backups (or in your case your boot drive) can also have hard failures at any time and one must plan for that contingency. (By the way, have you tried replacing the cable to the boot drive?)
I recommend NOT backing up to your internal drive. When or if the Mac dies you will not have straightforward access to that drive. It is also old and maybe past normal life expectancy as well, and it is very slow as well. Not a robust way to back up files. Backups have to be bulletproof. I would obtain another drive(s) with a reputation for reliability and connect with top end cables to make those backups asap before doing anything else.
