Getting content from SMBs is like pulling teeth...
You're never going to be able to get a single client to send you content. At best you'll need to have a sit down with the client and tell them the keywords and marketing concepts you are attacking and get the content written by your writer and allow them to "skim" over the end content after it's up IF at all. If you wait for approval they'll take till the last second to approve it. And god-forbid you get the client that thinks they know grammar better than you and doesn't understand what an Oxford comma is and looks over your shoulder with every piece of content.
I personally would walk through all content for my own brand, but that's because I've got a specialized service - so convincing me to give you content makes me think "why am I paying you if I have to do the writing?" even though I want to look over the content that goes out on behalf of my brand.
Easiest way to deal with this is to take over content creation and explain to each client the strategy you are going after and that you will be posting on behalf of their brand. You send them weekly reports and links to the different pieces of content that has been posted, social media posts, and other material - and after 3-4 weeks of this they'll trust you based on your judgement. ALWAYS ask for criticism and explain that it's critical so you can pass it to your writers and designers.
Also allow the client to send you blog posts and other things they wrote at anytime they want - that you then simple edit and post up. That's the "from the heart" scenarios that's great to mix in with your "SEO friendly" content. I'm not even a big believer in "SEO content" I'm more for educating the client's customer more so, but I dunno these SEO guys concentrate on keywords a bit too much and not the "heart" of the idea I guess.
Your marketing campaigns' budgets must include you creating content on behalf of your client - no way a client is going to mandatory write 2-3 articles a week, leave them the option to blog post once in a while, but you MUST take control of the messaging since after all you are the "MARKETER". Approach the content as a marketing message and not a "SEO" perspective and explain the marketing strategy to them, allow them to add their input and adjust, but you should NEVER wait for a client or ANYONE which will hold up your marketing - cause 3-4 months from now they'll simply tell you "it's not working out" since they aren't seeing movement EVEN though they will admit it's their fault.
I already know - specialized niches, bla, bla, bla - realistically if people are visiting your client's website they are most likely newbies and don't understand all the words and nuances - so you are in a perfect position to create content (or rather your writers are) as you get educated, they can created educational pieces of content that relate to newbies that turn into customers for your clients. This relates to the
"talking over your client" discussion we had - since your client writing content from an expert perspective would be talking over their customers too.
That's one reason I go in with a "marketing control" versus "SEO help". I can control the overall messaging across multiple online mediums which they can't do, hence why they are even hiring me in the first place. So it's a bit backwards asking them for content when you think about why they hired you.