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)