A 2011 iMac is a VERY old machine, the last verrsion of Mac OS that can it can run is 10.13.6. You may want to consider replacing it for many reasons including parts availability is nill as it was classed vintage serveral years ago now. You will also continue to have problems with software compatibility. You may want to consider a M1 based Mac Mini, for less than $1500 you can have a new M1 based MM and an external display and you are in business. Those machine far out perform a 2011 machine and can run current versions of Mac OS. A M1 MM is $1299 with 16GB of RAM and a 1TB SSD, all you need is an external display and you are done. External display begin at $100 so less than $1500 you have a very nice solution.
Your 2019 iMac cannot be backdated to High Sierra, you cannot install a version of Mac OS earlier than what the machine originally shipped with. For example a 2021 iMac that shipped with Big Sur, cannot run Catalina or earlier.
When it comes to technology it pays to keep your software and OS up-to-date. Also, consider replacing hardware every 6-8 years otherwise you will continue having issues. Sorry, not exactly what you wanted to read but by following these general rules of thumb it avoids issues.