iCloud/Calendar (desktop, iPhone) Sync Issues

As a final test with my issues before posting this, today I entered a calendar event in iCloud, and had it successfully sync with/appear on my desktop Calendar app and the Calendar app on my iPhone. The event is in a shared calendar; my partner also received this test event on his devices. So we have "downflow" FROM my iCloud Calendar.

However - despite everything working fine in the past, my desktop Calendar app no longer appears to have "upflow" to iCloud calendar. All events which previously appeared in iCloud Calendar and my iPhone Calendar (and my partner's) - those which are still on my desktop calendar - no longer do.

This issue has existed for a few months. Until I entered in the test event today, the iCloud calendar and iPhone calendar had gone completely blank, though my individual calendar names ARE still appearing on both.

I have ensured that all specified calendars have checkmarks on all devices; I have reassigned the default calendar on all devices; I've refreshed the calendars on my phone. Where I have had access too, I've set to "Push."

I am running Mojave on my desktop (due to third-party extension conflicts in Garageband, I can't update at this time.) Our iPhones are both up-to-date with system software. We have checked both of our Apple accounts/iCloud sign-in information; our devices all appear in our iCloud accounts.

Any help you could provide would be much appreciated. We rely primarily on our synced Google shared calendars; they show up in my desktop Calendar app. But we'd prefer to use Apple's Calendar moving forward, IF we can get it to work as it used to.

Could the Google sync be creating a conflict for the upflow from desktop to iCloud? This didn't seem to affect its operation in the past, and we haven't altered anything...

Is it because my desktop OS is not current?

Or are others experiencing similar issues?