Ok, then in that case I think you're on the right track. You definitely do not need (or want) to throw that out the window. I think I'm exactly where you are, especially because it sounds like it works for most customers. Do I have this right until now?
The challenge you face is that you have some (one or two, a few, whatever) clients that need MORE. Yes?
Cool. For those clients, move them to a vps configuration. What TrentaHost said is right. You want managed vps. Many hosting providers offer it. How is the host you are with now? Happy with them? Good server on time, good support? Next, see if they offer managed VPS. If not, you'll have to find out who you want to use.
You * only * want managed vps. You DO NOT want to be a systems administrator. And that's exactly the part where you feel "scared" and you're smart.
The managed part * of the managed vps means that it depends on the host for patch management and maintenance, keeping Linux and Apache and the bases and cpanel yyandyandyandyand already updated and running smoothly. It should be basically transparent to you just like your reseller account now. Period, complete stop. That is exactly what you want.
So leave the "small" accounts where they are, make sure your VPS is managed, make sure you have cPanel in them, add them as a server in your WHMCS installation. Then you simply provision them from there, basically identically to how you do it now.
The VPS will cost more, you may have to pay the cpanel license separately (do not skip, it's worth it), but if these customers are growing and need / deserve more resources, there should be no problem in including them in their costs. Money on it too.
You can possibly use a VPS like this for more than one customer. 2, 3, maybe 4. It's really just a simple question of shared resources / resources and for you to discover. If the VPS is not working hard, you can probably add another client. Monitor the performance and decide like this.
This configuration must scale to add 1 vps or add 100.
Congratulations for the growth. Good for you.