> I have to secure a physical object that grants irrevocable ownership of my wealth
That's one way, but not the only one.
My preferred mean of storage is through a _vault_ smart contract, such as the "Gnosis safe".
It's basically a smart contract that you deploy, and send your assets to. You can then add some of your trusted friends as co-validators and require 2/3 validations for a withdrawal. You can also set a no-validation required threshold at $X/m for the day to day.
> I obviously don’t expect them to guarantee the value of the crypto, just that the broker doesn’t lose it
For the record, brokers rarely (if ever) store or guarantee client money.
Usually you have a custodian to hold your money (who will guarantee deposits, up to some limits), and your broker will unlock a credit line for you based on your collateral posted at the custodian.
There are custodians in crypto as well, "Paxos" and "Coinbase Custody" being the most well known. They will store your money on cold wallets with very strict guarantees (shamir split of the wallet keys among anonymous holders, strong entropy guarantees on key generation, proof of reserve, etc) and unlock it upon verifying your identity with real humans. It's costly though.