I have no affiliation with Njalla, other than as a customer. I am always looking for more options for this class of service; but everything else in my current awareness for fully anonymous domain registration is either even more expensive than Njalla, or of unknown reputation. For my part, I welcome suggestions.
I didn't know Njalla, too bad that this site did not exist 10 years ago
There are also Anonymousspeech (or a name similar) and Katzglobal working similar but I just know by name. Never used and don't know their reputation.
Perhaps autistici.org too. I just use their email service but I remember something similar.