I use Google Drive for RStudio projects rather than OneDrive, and used Dropbox in the past. The selective sync feature for Dropbox dealt with the .Rproj.user folder unti it didn't anymore. When I switched to Google Drive I purchased a license for insync software and set an Ignore Rule for that folder. All has been well for several years now. Their web site is www.insynchq.com. It works for OneDrive, Google Drive and Dropbox and runs on macOS (finally native on Apple Silicon!), Windows and Linux. I have a grandfathered lifetime developer license that includes the required Ignore Rules feature.