Couple of ideas to look at:
Try turning off W3 Total Cache plugin. (after the last Avada update, WP Rocket, which is the cache plugin I was using, started causing problems: parts of the site would appear, disappear, return. I turned it off, and the problems went away.)
Reset Theme Cache, Browser Cache, Server Cache And Plugin Cache
Click on registration and confirm your theme token still has a green check. Sometimes it will disappear.
Check the settings in the server’s htaccess file. It there a setting there that is telling WP to access the earlier version of PHP.
Check the error log on the server. Is it indicating any PHP or DB errors.
If the system status is still showing PHP5, call your web host company.
Confirm you have only one Avada folder present in the Theme folder.
If you’re using a child theme, switch to the parent theme. Does the problem still exist?
On the parent theme, wwitch to the child theme of Avada; login to your themefusion account. Click on Avada Plugins. Download Fusion Core and Fusion Builder and reinstall through the plugin upload of WP.
Try selecting a completely different and generic theme. Activate it. Review just homepage. (Scary I know.) Go to themes again. Choose Avada child. Activate. Review homepage. Everything should look back to normal.
Find the date of the last working Avada, and restore site to that date. You may loose a few registered users but it’s better than 600.
Review all of the installation notes that start here: https://theme-fusion.com/documentation/avada/install-update/avada-speed-performance/