you don't need to store cryptocurrencies "for" clients because unlike fiat, transferring cryptos is easier, fast and a lot cheaper. and that would be also a lot safer. since you already claim it is to "buy and hold" then you must ask for their addresses and pay them right away. then it is their responsibility to learn how to properly secure their coins.
if i were you i would worry about more important matters such as getting involved with altcoins that can get dumped hard since they are pump and dumps even though they may seem like a big deal!
Thanks for the response but where in my post does it mention other altcoins? I am not interested in shitcoins. Seriously though, my client really isn't interested in learning about bitcoin - that's why I made my post.
if your first paragraph when you say "I buy BTC and/or ETH", in case you didn't know, etherum is an altcoin.
as for your clients not wanting to learn about bitcoin then why are they bothering with it? you might as well invest in anything else and only give them the profit. so as a result you might want to change the whole idea and cut their involvement altogether. for example only sign a contract that obligates you to give them profit per month regardless of what you do with the money!