Hi Sue,
Office 365 subscribers can choose from three channels:
Current Channel
This channel provides the general release that is what most people should use. It is fully supported. An update comes out every month, generally during the second week of each month
Current Channel Preview (formerly called Insider Slow)
This channel provides the general release build a week sooner than Current Channel. The main purpose is for IT departments to get the upcoming general release a few days early and test it to make sure there isn't something that's catastrophically wrong. IT shops can control updates and can delay installing the release should they find a problem. This channel is also for people who have a known issue and the update is known to have a fix for the problem. Their troubles go away a week sooner if they install the Preview build. It's also for the people who always want to be first in line.
Beta Channel (formerly called Insider Fast)
Beta channel builds are weekly, raw, unsupported builds that Microsoft releases for testing new features and bug fixes. The feature set can change weekly. There can be crashes and problems, which Microsoft hopes the beta testers will report and comment on, and Microsoft wants feedback on new, yet to be released feature experiments. These builds are not officially supported.
Note that the forum channel names still have not caught up with the in-product names of the above 3 channels.
To switch from one channel to another, in an Office application choose Check for Updates, which launces AutoUpdate app. Click the Advanced button and then click the pop-up to choose a channel.

If you switch from Beta channel to a different channel, when that channel's update is newer than the beta channel build, you will then get the current channel updates and will no longer get beta channel updates. It will not instantly revert you to current channel builds because doing so can cause major problems, as I mentioned earlier. Not only can Outlook become non-operational, dates and times can be messed up. It's best to just wait till the current channel build overtakes the beta channel build.
Running in safe mode was a good test. When you boot to safe mode, only the system fonts and fonts that shipped with Office are active. This can help rule out problems with your additional fonts. In this case, your problem did not go away in Safe Mode, so fonts are probably not the root of the trouble.
Testing in a new Mac OS user account would be the next troubleshooting step to try.
If there's no improvement, then reinstalling Mac OS would be the next step to take.