If you want to follow Lifeisabeach's advice and I'd certainly try ferrarr's method first a "clean instal" is a good idea every once in a while.
If you wish to try the clean instal here is how I do it.
Download the current Catalina installer from the App Store, quit it once it's loaded because it will mount and attempt to run.
Download the free app Install Disk Creator from
https://macdaddy.io/
Use it to create a bootable installer on a USB thumb drive. About 10Gb will be plenty. It will erase the drive so you'll need an empty one.
Download a trial copy of Carbon Copy Cloner (CCC) from here,
https://bombich.com/
Use CCC to clone your hard drive to an external USB HDD drive or SSD if you have one. Check after completion that it is intact and bootable.
Eject the CCC clone (there will be two drives; Macintosh HD - Data and Macintosh HD volumes) make sure you eject ALL.
Now you can boot your device from the thumb drive. Simple hold Option key on boot and select the thumb drive.
In the installer go to Disk Utility and erase your Macintosh HD - Data and Macintosh HD volumes.
Proceed to instal Catalina from the thumb drive. Follow the prompts. Bear in mind it takes a while, estimated completion time is only for each stage and the device will shut down intermittently throughout.
You should have a mint device at the end of this process, just like a brand new Mac. You can now boot from the internal drive and remove the thumb drive.
Connect you external HD and import the data, files, apps by drag and drop. The CCC clone will look just like your old HD.
Personally I keep this process to a minimum.
The first thing I do is drag the contents of my Documents folder across.
Set up my email accounts. I use Spark so I download a fresh copy from the App Store. You accounts data will download from iCloud. Apple Mail will need to download all of your email again.
Your Safari browser will retain all of the data it had before if it was synced via iCloud, which is the default. Other browsers assuming you had an account, eg Google Chrome will also retain your bookmarks, passwords etc so you can download fresh copies and login. Where possible I download fresh copies of all my apps except simple free standing ones. This reduces the chances of corruption or missing files.
Initially you wont need much. You will have all your contacts, documents, browsers, bookmarks and email services so you can take your time downloading or transferring the rest.
DO NOT sync CCC just keep the drive on hand so you can find old stuff if or when you need it. For me that takes about a month to be sure I've got everything.
The above process takes 4-6 hrs much of which can be completed unattended.
I would set aside a day to do this, maybe on a weekend. Ensure you have all of your app passwords usernames etc on hand and remember if the whole thing goes sideways you can always simply reverse clone your original contents back from the CCC clone onto your device and it will be just the same as it was before you started.
Note; you will have a copy of the Catalina installer in the App Folder of your clone. This may or not be handy at some stage, just FYI.