There's no ideal situation here.
You can do 2-of-2 and hope that your client never loses their key/seed, but if they lose them, the money is gone.
You can do 2-of-3 where you hold two keys (in separate physical locations to prevent compromise) and your client holds one, but this effectively places the funds in your sole custody. That raises liability issues if the funds are ever stolen.
You can do 2-of-3 where you hold one key and your client holds two (in separate physical locations) but again, this opens the possibility that your client has his keys/seeds compromised.
Whatever you do, I would avoid any liability in case things ever go awry. You don't want to be on the hook for a 20 BTC loss.
You could also consider
an institutional custody service like Bitgo. I'm not sure exactly how their setup and pricing works but lots of services use them for multi-sig storage.
This was my point and OP I hope you pay attention to this. Forget all the complexities of multisig and all the litigation risk plus extra vulnerabilities of custodial service (meaning, you should never, ever take care of their bitcoin for them).
You've repeated it that you only want to manage it for them, and advising them to hold. So a simple hardware wallet, seeds owned only by them. You can get them to store the seed in their safe/vault/wherever they store their most precious thing. They can even write it down along with their Amex for all you should care, that is their choice and their risk. Tell them of their responsibilities to seek their own custodial services if they don't trust themselves.
You simply don't want liability, why spoil a great and profitable relationship?
Hardware wallet. You know the address, that's all you need to manage and report. Good luck!
As I have already said, a hardware wallet is not good enough.
Think of it like this. I set up my cleint with a hardware wallet. At the end of February, I convert Jan & Feb profits to BTC. So currently it will be about 73.5BTC.
Client gets phished/scammed/ wrote his seed words somewhere stupid.
BOOM thats 73BTC lost. Brilliant.
So a hardware wallet is NOT good enough for this.
"Tell them of their responsibilities to seek their own custodial services if they don't trust themselves." Thats what he is paying me to do!