I'm a relatively new academic writer, very much enjoying it so far. I went the route of working through a company rather than going solo - so far, at least. I thought I'd share my own considerations for whatever value they may have for you. I don't have any experience operating one of these websites (and it seems some of the people on this board do), though I do know a bit about website-related matters from my own hobby interests.
Now, assuming you're just interested in going solo-solo - that is, not in starting a company with other writers as well - it's a bit of a different situation than it is for most of the websites people are discussing here. We're talking about something that is going to be more of a personal-freelancer sort of site. If you decide to give it a real try solo, that's definitely something you'll want. Most everyone in any industry who works freelance full-time has their own site nowadays; it's mostly a 'personal branding' thing showing that you are a professional, demonstrating your talents a bit.
The personal-freelancer website is something you put in your email signatures, list on Facebook, put on business cards, etc. Generally it is not the way that people originally find out about you - it's a way for them to check you out further after making original contact somewhere else. As far as Search Engine Optimization (SEO) strategies for people to find you on Google search.. I'd suggest you forget about that. Make sure people can find you if they search for your name, or whatever business name you might want to go by. But like you mentioned, there are unfortunately thousands of sites that show up for "academic essays" or any similar search term. It's possible in principle to outrank them, but even if you managed to do it, it wouldn't be worth the time and the expense. The "big" sites are places that handle tons of orders and employ many writers. As an individual, the volume you'll be handling just doesn't require the same strategy. Besides, what would happen if you suddenly landed your site on the first page of Google results? You'd probably be swamped with hundreds of requests every day. That's not really so great when you just want to focus on writing.
So I'd suggest, if you want to go solo, that you will have to do some active self-promotion if you want to find clients. If your work is good, eventually you may get a lot of work from repeat clients and referrals - but starting out will likely be a lot of effort to find work. I'd also recommend setting a very firm pricing policy, and make it prominent on your website and clear in any posts/communications. You don't want to waste your time talking to a bunch of students expecting to pay $10 a page and have 24/7 phone access to you. Check out what the other websites are charging and match it, or charge more. You'll have to account for the volume of work you can actually get, not just the pay when you do get something.
All of this is why I decided against the solo route, for now at least. As long as you can find a decent company that matches your own ability, it is in my opinion a nice way to go. Yes, they're probably going to pay you 35-50% of what they're charging clients. Personally, I'm more than happy to let them have it. I don't have to spend my own time and money advertising. I don't have to deal with indecisive customers on the phone or through email, leading me on for a week and then deciding not to hire me. If you've ever used Craigslist to sell off old furniture.. remember that experience, lol! For every item you actually sell you get 30 email inquiries. Half of them never reply when you respond. The other half go some length toward setting up an appointment, but half of those either cancel or just.. don't show up. (Sometimes the ones who express the greatest confidence that they really intend to buy it.) Of those that do, half of them decide against buying, and most of the others pull the "Oh, your ad said you wanted $250 for the sofa, and I know I didn't ask before.. but how about $100?" routine.
Well, okay, it's not always like that. Being very clear and firm about prices helps to cut down on it. But you get the idea. As for myself, I'd be happy to deal directly with clients who had already paid. Dealing with potential clients, however.. I'll take half the per-page pay and just get directly to the writing.