The best for you would be to make a clean install of macOS.
First backup all you personal data (docs, songs, picture, etc.) to an external disk.
Create a bootable USB stick with Catalina Installer https://support.apple.com/en-us/HT201372
Reset SMC - https://support.apple.com/en-us/HT201295
Reset NVRAM - https://support.apple.com/en-us/HT204063
Turn on your Mac holding Option so you'll be prompted with boot options
Choose the Installer
Once you get to Recovery Mode, erase the Mac disk completely https://support.apple.com/en-us/HT208496
Start the installation of macOS and go through the entire process.
Once macOS is installed, check for updates and install them all.
After that install all your apps.
Now start copying back all your personal data from that external HD.
That way you'll have a clean install and the best experience with your Mac.
I do not recommend using Time Machine or Migration Assistant in your case. You don't want debris from previous system like old and/or corrupted files and settings that might cause you problems. This is even more important here because you're still running an older version of macOS.
Take your time to do that clean install and you'll be very pleased with the result.
