I can see two possible strategies:
=== 1. Fully separate accounts now ===
What I said originally - set up separate accounts for each of you customers and have them pay for them themselves. Have each give you access to either their account or a user under their account to author and maintain their website.
=== 2. Contingency Plans ===
Make contingency plans for your clients in the case of your abrupt demise. Definitely have them own their own domains - you can ask DH support whether you can have them have their own accounts for domain registration but still host all the domains under your account here. If that's not possible, have them own the domains at some other registrar and just host the domains here.
Next, make sure they have backups of everything. The easiest way of doing that is probably to set up each of your clients as separate users under your own account. That way, you can just give them the username and password for their account and have the take the backups and do transfers themselves.
Alternatively, you could make sure that each of them has a place where you could send backups of their sites to them.
I'm sure the other kind folks here will have even better solutions!
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