I've had the same experience. Signed up for Advanced Protection Program not long after it was announced and I've been very happy with my MFA experience with Google for years now.
If you use a Google Titan Key with Apple devices, have you been able to pair and successfully authenticate using either (1) bluetooth only, or (2) NFC only, on any Apple device?
I have not, and it's frustrating that Google offers no support beyond their help pages for their own branded hardware.
I use a few Yubikeys and a Bluetooth Feitian key with my iPhone 8. My wife uses a Google Titan USB key and Bluetooth key for her Apple devices. Our iPads are too old to have NFC, but the Bluetooth paired fine with Google's Smart Lock app and we've used that.
For our iPhones, the NFC has worked fine even without the Smart Lock app. I believe Google phased Smart Lock out some time ago but I've not tried using it in a while, I don't use an iPad very often.
Yes, the bluetooth titan key definitely worked for me and my wife on our iPhones. I think NFC did too but I haven't had to reauthenticate my iPhone in ages so I don't remember for sure.
I have not, and it's frustrating that Google offers no support beyond their help pages for their own branded hardware.