[eru]

I would definitely trust my lawyer with my bitcoin seed.

But the whole thing depends on how much you own in bitcoin.

If it's a whole lot, check how other people in more traditional domains are dealing with their lawyers or notaries handling these sums. (For one, it's a bit easier with bitcoin, because you don't need to tell your lawyer or notary what you are giving them. And you can encrypt the private key data with something derived from an easy to remember password. It doesn't need to be 100% cryptograhpically secure, it just needs to lower the temptation for your lawyer.)

Btw, I think the bigger problem in practice wouldn't be your lawyer stealing from you, but your lawyer somehow losing your data.