iMac late 2015 keeps restarting - Using macOS on External SSD

I believe your internal hard drive is most likely causing the problems. The Kernel Panics listed in the EtreCheck report mention "AHCI" which is the controller interface for internal SATA drives. Second, your internal Seagate Hard Drive is showing up as a 4GB (that is a G), but that drive should be a 3TB hard drive.

Kernel Panics (and that is what is causing the restarts) can be caused by either hardware or software. My guess is yours is RAM related or a failing drive such as the hard disk inside the iMac. On such an old machine I would STRONGLY consider replacing it and not spend time on a vintage, obsolete computer. It's time to move on instead of trying to live in the past.

In the event that you were not aware, if you purchase a computer directly from Apple they offer 12 month same as cash financing. So if the cost of the new computer is $1500 (I just used that number as an example) you can make 12 monthly payments of $125 and you will pay it off in 12 months. So instead of saying you cannot afford it, do some research and maybe you can and just don't know it!

You have a "Vintage Diesel Engine" , (your Imac 2015) ,using some new spare parts.

External SSD Harddrive on a Vintage 2015 iMac!

Kernel Panik will occur at some point,on this Vintage iMac!

HWTECh and R:Kaufmann, i agree with on this issue!

Purchase a new iMac or Pc?

Understood.