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.