If LE goes down for a week you can't deploy new certs, but your existing ones will work, as you renew them a few weeks before expiry anyway
That also gives you enough time to change to get your certs from elsewhere
As you mention zerossl exista, and I think google GCM will give you free certs too.
Globalsign has an ACME interface for paying customers, although I'm told it has issues (you have to rotate keys manually every X days / N certificates)
There's no (current) plans to drop below 45 day certificates with an expected renewal with 2 weeks to go.
I agree if cert lifetimes drop towards week long then it becomes problematic. A sensible thing at that point is to ensure you can issue certificates from different CAs on different underlying stacks, in the same way you use multiple DNS servers
That also gives you enough time to change to get your certs from elsewhere
As you mention zerossl exista, and I think google GCM will give you free certs too.
Globalsign has an ACME interface for paying customers, although I'm told it has issues (you have to rotate keys manually every X days / N certificates)