iMac 14,2 on Catalina crashing after reinstall

You aren't going to get a "it's XYZ and you need to do ABC to fix it" type of answer because the computer is suffering from Kernel Panics. KP's can be caused by hardware or software and you have to figure out which. You can start by providing a report and we can look to see if there is anything obvious causing your KP such as a failing drive or using inferior upgraded RAM or even worse, having third party antivirus, cleaning or other types of apps installed.

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click How to use the Add Text Feature When Post… - Apple Community It is helpful to us if you download the report to your Desktop.

By the way, if you cannot download and run EtreCheck on a regular startup, please do so in Safe Mode

Well this does not have an impact on your KP problems it MUST be addressed. You have all of your data at risk and with an 11 year old obsolete computer that is really really risky. It is at risk because there is ZERO evidence that you backup. This is a 100% guarantee you will lose data that may be unrecoverable. Operating any computer without backing up is playing Russian Roulette with your data. At the very least you should be using Time Machine to backup the computer. If you are not familiar with Time Machine please click Back up your Mac with Time Machine to learn about it.

What I see is you upgraded the RAM at some point, please remove the upgraded RAM and run the machine for about a week and see if the KP's continue or not. If they stop, then you have found one contributor to the KPs. That means, replace the upgraded RAM with high quality RAM from either Crucial.com or Macsales.com. If the KP's continue that means the RAM is not at fault. IF the upgraded RAM was Crucial or Macsales.com then contact the vendor as their RAM is covered by a lifetime warranty. First though remove and reseat the RAM and test to see if it simply wasn't seated well, that can cause KP's too!

If the KP's continue with the RAM removed then I suspect the internal HD is failing. I have seen you have DriveDX installed, run it on the internal HD (NOT the Fusion Drive or the internal SSD just the internal HD) and if the DriveDX report has any errors, then you have found your problem, it's the internal HD and it must be physically removed so you can run reliably on the external SSD. If the internal HD checks out with no errors then run Drive DX on each of your drives and see if any errors pop up.

Also, please ensure all third party software is up-to-date, out of date apps can cause KP's too.

Finally considering this computer is almost 11+ years old, please consider replacing it. It's old, obsolete (won't run current versions of Mac OS and parts are becoming difficult to source) and has seen its day. You have a lot of options available to you, it doesn't have to be another iMac if you don't want, consider a Mac mini or an MacBook Air or MacBook Pro or even an Mac Studio if budget and needs make sense. All of the above would be a huge upgrade over your 11+ year old iMac.

The last thing I can think of is you may have damaged something when you opened the computer, in that case it's definitely time to replace the machine.

Yup, the internal HD could be the problem. If you are not using it, physically remove it!