What are the viable alternatives to LE? And in case none exists, what does it take to build one? Requirements: free, available to everyone, automation friendly, issues certificates that are actually considered trustworthy by other parties.
None. Big tech intentionally made Let's Encrypt a single point of giant failure.
> And in case none exists, what does it take to build one?
A new Internet and Web standards stack. The whole problem is self-imposed -- we could have published self-signed Ed25519 keys on the DNS instead, and the result would be more secure than whatever it is we have now.