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.
I don't want to be responsible for their seed words. Plus I need a safety net in case my client ( who isn't technically minded) gets phished or dodgy browser extensions or malicious software on his computer ends up stealing his coin. I know using a hardware wallet its unlikely BUT.. I guess I am saying I don't trust my client to keep his seed words safe for himself and want to make sure there is another step required to move his coin.
he isn't technically minded enough to deal with firmware updates etc etc. One of the big factors for this to work, is to minimise any work required for him. It needs to be simple and straightforward from his perspective, even if its more technical behind the scenes
I have been working with one client for over 5 years, and we have a great trust relationship (and I earn a shit load of money for him and me) I wouldn't want to blow that trust (and him lose money/coin) just because he isn't technically minded enough to understand about keeping his seed words etc safe. (This is the type of guy who has his Black Amex card PIN written on the back of a photo in his wallet !!!!!)