There are no custom / additional types yet, so secure notes are the way to go for now if you want to store it in Bitwarden.
If you want more structured data, you can use custom fields to separate seeds, pins or whatever information you want. You can even designate the sensitive info as a “hidden” custom field so that someone standing next to you, when viewing your public key can’t see your seed text for example.
Unless I’m remembering incorrectly, I believe there is a 5000-character limit for the encrypted cipher of a custom field (compared to 10,000 characters for a secure note), and the encryption process typically expands the field contents by 20-30%. Thus, in some cases, a long cryptographic key may not fit in a custom field.
I would never recommend storing seeds for any blockchain cryptocurrency wallet of value in a cloud-based password manager. Large wallets are susceptible to targeted attacks by very determined and skilled groups of hackers. Offline seed management in a secure (water; fire; theft) environment with redundancy. Especially for NFTs, the current practice is to keep your main wallets in cold storage, only temporarily transferring in the amount you need for a transaction to a hot wallet so if the hot wallet is compromised, you only lose the amount you temporarily transferred. The main wallet never does a direct transaction for any purpose. Would recommend Ledger as one cold wallet storage solution. But, again, the Ledger’s seeds should be kept offline in a secure environment with redundancy and should never be entered electronically anywhere. Just my .02 cents.