My Galaxy Tab S7 updated on July 10th. Since then the Business Calendar 2 app no longer syncs with the calendar on my Galaxy S10 phone and vice versa. I tried using the Samsung calendar and have the same issue. The only calendar app that will sync on both devices is Google's. I have uninstalled and reinstalled the calendars, cleared cache and data, checked sharing settings, etc. without any luck.
Sounds like you've covered all the troubleshooting all I can say is default apps disabled other calendars to let your other calendar business calendar to app be the default career catch clear data and apps settings reboot device you know the protocol