I’d just get a shared account for 90% of them & a woocommerce managed plan for any that require it — direct via automattic is probably the most sane route, albeit most costly.
Avoid reseller. Avoid anything dedi., focus on your core business.
If they’re ok having their client sign up for a provider or signing the client up and transferring the account it would take them out of the middle between the provider and the end-user as far as support.
It also depends on whether they want to bill the client for hosting and make some profit on that or simply offload it 100% and only do whatever else it is they do for their clients.
With a quality provider, the issues they’ve faced on the dedicated server generally shouldn’t happen and, if they do, it would be up to the provider to resolve them regardless of whether they are having their clients on individual shared accounts or accounts within a reseller account.
Personally if I were them I’d go with a reseller – but they can obviously go the route of individual shared accounts if they want.
I absolutely would not recommend putting multiple clients within a single shared account – even if they don’t access the cPanel. You lose isolation between the accounts. Just because they don’t access the cPanel does not mean that there is no way for them [or their site if they get hacked] from accessing other users / data.
They could, for example, easily find other configuration files within the account and access data that is not their own.