So far it seems like we've been very lucky, 70+ websites updated and only one that had a problem with the theme builder not loading, luckily that particular site is fairly static so we dont have to rush and find the cause.
This is our process of updating :
1. Update all WP Core and plugins except
Elementor + Elementor pro
2. Update Elementor 3.0.1 and Pro together.
3. Goto Elementor - settings - Advanced - Enable
Optimized DOM Output 4. Goto Elementor - tools - Regenerate CSS
5. Goto Elementor - tools - Sync Library
6. Clear/purge any Site caching plugin installed
7. Clear/purge any hosting or DNS related Caching ( such as cloudflare )
8. Hard refresh on frontend to ensure all site elements are working as expected .
9. Go back to Admin panel, click on theme builder. Most of the time you are presented with an error, ctnl F5 ( hard refresh ) and the new theme builder should load.
Do final checks and all is fine.
The only site we've come across so far with an issue is one which someone else setup, installed a bunch of plugins. I suspect the biggest problems people have are not with the updates themselves, but their 3rd party reliances which havent got around to updating.
****Disclaimer, please note the above process is simply what we do because we know our custom themes, frameworks and use minimal plugins, follow at your own risk. Always have a backup.