A good backup including the database… And then go for it.
You might install https://www.ads-software.com/plugins/wp-downgrade/ but you don’t need to enable it right now.
If you do need it and can’t get to it to enable it then it probably wouldn’t help you anyway… just restore the backup instead.
But you might use it to revert back to 4.9.9 to give your client back the interface and editor they are used to if all goes well.
I imagine you know about the Classic Editor plugin which will let 5.X run with the old Classic Editor and optionally the new Block Editor also concurrently if you wish.
https://www.ads-software.com/plugins/classic-editor/
Then I’d recommend adding Health Check and I’m recommending the Wapuu Dashboard Pet along with Health Check now.
https://www.ads-software.com/plugins/health-check/
https://www.ads-software.com/plugins/wapuu-dashboard-pet/
Wapuu is a little bit of ‘whimsy’ but it reacts in stages to missing updates and other issues and now it can be set to remind the owner/staff to add content on a configurable basis. I expect it to get better.
Seeing that little character in the dashboard footer makes me smile. And if your client doesn’t like it you can always blame me then remove it if they want.