"Your security key does not have enough space for any more accounts" - What do you even do in this situation?

I have a lot of accounts but most don’t accept passkeys. If you need a very large amount, a device with a TPM and fingerprint reader could be worthwhile. This is a common feature of phones and laptops. You could still use yubikey for the most important accounts. You can also get one that accepts more than 25. YubiKey 5C NFC says it accepts 100. If you had gotten 5 of those, that would be 500 accounts. That’s around how many I have in my password manager but, again, most accounts don’t use FIDO2.