I work as a consulting engineer, and given past experience, I'd politely decline (or, perhaps, renegotiate) some of the jobs you're proposing to take on.
The problem is that, when you deliver a prototype (or schematics, or layouts, or whatever), you're implicitly responsible for making sure it works. If it's completely your own design then that's fair enough, but the difficulties start when the customer's supplied materials have errors in them, or just don't work the way they expect.
This means you end up having to carry out a full design review of everything you're supplied. It could be that you're given a perfectly good prototype which really does "just" need laying out on a custom PCB - but, equally, a design you're given to work from could be a disaster in terms of reliability, or EMC, or production yield, or cost, or technical performance, or whatever. This becomes your problem as soon as you produce a derivative product that inherits the same problems, and you'll end up taking the blame for not solving them.
Bear in mind that if your customer understood those problems in the first place, you wouldn't be having to deal with them. You may find yourself having to explain to someone that their own work is bad, and that's all but impossible to do diplomatically.
With that in mind, I have a policy now that I work from specifications, and I do the design work myself. This means I'm only responsible for my own choices, and inevitably it's quicker and more reliable in the long term if I'm developing and maintaining something that's done exactly how I'd have done it, and which has no unnoticed 'gotchas'.
All my customers are businesses who have manufacturing capability, but not electronics expertise. They hire me on a regular basis whenever they need something electronic designed, debugged or modified.
I like it this way; we all work well together, and everyone's time is used efficiently. If you deal with too many people each of whom only wants a one-off, you'll find yourself spending a lot of time and effort dispensing free advice.