iMac Big Sur restarted more than three times a day

RAM should always be installed in pairs, and 3/4 is not in pairs. Look here for the proper RAM: OWC Memory Upgrades For 27" iMac with Retina 5K Display (Late 2014 & Mid 2015) - MacSales.com (OWC).

However, I do believe that your un-authorized DIY internal upgrade may have caused this issue. I suggest you contact a local AASP (Apple Authorized Service Provider), have them remove your DIY components, then purchase an external SSD to run macOS with: How to Setup and Use an External SSD as y… - Apple Community.

Resources:

Find an AASP - https://locate.apple.com

Cheers,

Jack

Welcome to the Apple Support Communities.

I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Jack

Time Machine backup out-of-date - The last Time Machine backup is over 10 days old. • Make sure your backup is up-to-date!

Kernel panics - This system has experienced kernel panics. This could be a sign of hardware failure. • Please see: If your Mac restarts and a message appears - Apple Support.

Also, did you upgrade the RAM yourself? If so, what brand was used?

Cheers,

Jack

Your EtreCheck Pro report only shows 3 8GB modules. If you have 4 then either one has failed or one is loose.

First I would recheck to make sure that all of the modules are fully seated in the slots.

Then if it still panics, I would test 2 like modules and then test the other two.

If I was going to buy more modules, I would buy 2 more of the Crucial modules, so that all 4 matched.

You're welcome. Let us know if that works.

Also the Apple Diagnostic Test, might be beneficial in this case.

see > https://support.apple.com/en-us/HT203747

What RAM modules are you using and why only 3 instead of 4?

https://eshop.macsales.com/shop/memory/iMac/Retina-5K/DDR3L

For a backup drive, get a high-quality, desktop-class 7200 RPM external drive, such as an OWC Mercury Elite Pro - MacSales.com (OWC).

Make sure it is a 8GB Module.