- As another user has posted, is the 5400 rpm drive part of the problem? Would a SSD help greatly?
Absolutely (huge difference).
Professional installation is highly recommended, especially since you may still be covered by Applecare if you bought it. Here are install videos:
https://eshop.macsales.com/installvideos/iMac-21-inch-2019/
Or, less expensive, take a look at above vendor and get an external SSD in an enclosure and use that as your every day boot drive. You can simply format it in Disk Utility, download CarbonCopyCloner, and have it create a bootable clone on the external SSD. You can then boot and work from it. Once you know everything is running perfectly, you can use the internal as a backup drive (after wiping it).