cdflasdkesalkjfkdfkjsdajfd If I were you I would search the specific services you plan on using with the security key to find out about compatibility.
I blindly jumped into a security key purchase after reading about the lofty ideals of the technology without thinking of the practical implementation. I assumed I would buy a yubikeys and never type a password again, and unfortunately that's not the case. I've been underwhelmed by support from my most used services (commerce, banking, ect).
Another grapheneos-speceific consideration is that in order to utilize fido2 with web services, you have to use the webauthn interface, and in grapheneos that webauthn functionality is dependant on Google play services.
In my security key adventure I have found that TOTP has much wider adoption among services which I was hoping were compatible with fido2. If I were doing it again, personally I'd pay for a premium subscription to bitwarden, which has totp functionality for paying users, and then see if I still wanted the fido2 security key (personally I would, since you can then secure bitwarden with fido2). There are other free totp apps like aegis that are probably good, but if I'm using a password manager anyways it's nice to have that totp feature built in. That's just my perspective, there are other considerations of course.
Hopefully fido2 adoption will increase over time.
There's some good info on this thread:
https://discuss.grapheneos.org/d/10542-2fa-otp-app-using-the-titan-secure-element
And if you search the relevant key terms in this forum I'm sure you'll find more good info