Let's encrypt is a single point of failure for a large percentage of the internet.
However, thinking about how to make your own setup more robust without having to manually change configuration when one SSL provider stops working is a good exercise. I wonder if you can just get your server's private key signed by multiple SSL providers, and serve multiple certificates to clients, and whether all browsers handle that correctly.
If LE was to go nope right now, how fast could you move your stack from LE?
You can't use multiple SSL certificates as redundancy. You could probably create something bespoke with a Load Balancer and SSL offloading but that's just more overhead for really nothing.
I haven't had one in 20 years, which kinda proves my point.