Wait - if you are transferring them their BTC by the end of each month, why do you need to hold funds for them? I'm terribly confused.
But if you do need to hold funds for them, I think that they will obviously appreciate control over their funds to the fullest possible extent. Multisig in this instance could certainly work.
I still think that a hardware wallet should be discounted. I don't think that it's that technical intensive given ledger/trezor's intuitive UIs. In this case, you'd only have to worry about sending the BTC to the correct address at the end of each month, instead of actually holding a key in a multisig. They would have full responsibility of securing the BTC which I don't know if they would be comfortable with. Please do correct me if I got your scenario totally wrong.
Exactly my thoughts.
You want to advise your clients to buy and HODL. Please teach them to use their own wallets (ledger/trezor as magneto says, come on 5 minutes is all they need), I would recommend you charge them only to teach them how to use it, and perhaps to monitor their holdings using watch only, so you can provide full reports and updates.
You don't want to diversify into custodial services, not if you're doing this on your own. Multisig is a workaround but I seriously don't recommend it - why do you need control over their funds? Seems like a drawback to me.