There is no perfect solution to this, as there must be some  level of risk being taken by the person. It would have been lovely if such a person can keep the seed phrase all to himself but eventualities and death may happen, this is why it's good not to keep it all alone.

Bank's keeping is a good one, though expensive and you should never let them know it's the seed phrase, and you may keep them in part. A good law firm is another means to keep this, but like I said, in part, anyone can compromise.

All these trouble because spouses, relatives, or anyone may compromise, yet they will still save the day. So, just as you entrusted a part to a bank/lawyer, you can entrust the other part to anyone you trust most in your family who can link the pieces in case of an eventuality.