Hi Knitsalot
Welcome to the Insider forum. I moved your question from the general support forum to the Insider forum. When posting questions about Beta Channel builds please use the special Insider forum. I see that the current channel forum replies asked you to uninstall your beta channel build and install the current channel build.
Beta channel questions are answered by different people. Reponses in the current channel forums are made by people who generally are not familiar with beta channel builds or what's going on with them, hence the advice to switch to a build they are familiar with. I think they should have moved your question to the Insider forum instead of switching you to a current channel build for what I assume is their convenience.
Another forum convention is that you should discuss problems individually. Putting Excel comments into the Word forum means that the Excel folks won't see your Excel worries. It's free to make questions, so limit your comments to the application whose forum you're posting to and you'll get much higher quality replies. It's also a good idea to keep each question about a specific problem. You're OK on that point because all your comments involve how the screen is being drawn.
The whole purpose of beta builds is to expose trouble before it ever gets to the current channel or even Preview channel. I would expect you would not have been told to remove the beta channel build and then install 16.50 had you posted to the beta channel, which is currently build 16.52
It's generally not a good idea to install previous versions of Office. Doing so can make your Outlook profile unusable. You're lucky there hasn't been a recent change to the way Outlook uses its database. When going backwards, it is very important to completely remove your current software before installing the old version. I see that you removed Office before reinstalling, so that is good.
Because you have the same experience on three entirely different versions (16.50 16.51 and 16.52) it is highly unlikely the problem is related to Office software or installation issues. If this were a common occurrence, the forum would be flooded with other reports like yours. It's not.
There have been a handful of reports saying Excel gets slow when OneDrive application has been installed. Have you installed the OneDrive app or any other app that might be trying draw the screen while your Office apps are running?
Do you have a boat-load of fonts? Are you using any fonts not supplied with Mac OS or Microsoft Office?
Have tried running in a new Mac OS user account? If not, go to System Preferences > Accounts and make a new user account. Then, while in the new account, test Word and Excel and see if there's a difference.