Not quite true.
> every message has the recipient keyid in plaintext
The keys are not required to be centralized in any particular location. There is no way to tie a key to an individual, unless that individual wants to be associated with that key id.
It's common practice to post anonymous, encrypted messages on mailing lists or newsgroups. All you can really tell in those cases is that the recipient is a member of that mailing list or subscribes to the newsgroup (though it's not for sure, with the use of remailers, etc).
How large?
> several recipients
How many, in what countries?
Is the communication anonymous?
Can I use it in such a way that obfuscates the message's recipient?
Can I send an encrypted message when the Signal servers have been DOSed?
If seized, can the controllers of the Signal servers get the contents of my entire phone contact list?
I believe that Signal is adequate for a small subset of encrypted message cases, but not as a good replacement for encrypted emails.
I'm unaware that Signal is geographically limited in any way.
Signal is anonymous as your phone number is. PGP isn't anonymous either, so this seems like an irrelevant criticism.
Again, you can obfuscate the recipient as much as you can obfuscate a phone number. You can't obfuscate the email address you're sending your encrypted email to.
This depends on how they use contacts to match users. I believe they only collect a hash of the phone numbers you choose to share with them.