I know there are more mainstream poker sites that use bitcoin in addition to other methods, but I'm wondering if there are any more bitcoin-only sites that I've missed.
No, there would be very thin possibilities for bitcoin-only gambling sites to be existing. I mean not just poker gambling offering site, but in general with all types of gambling also, we cannot find a services to be accepting only bitcoins as these days altcoins have become a prominent options in gambling industry. I am sorry, I could not recall a gambling site which is accepting only bitcoins and offering poker.
Probably, you may look for a casino which is not accepting any kind of fiats including different payment processors. Because, when a casino handles fiat then it may start enforcing us to clear KYC at any time and most gamblers still hate it.