If you don't have a 'trusted wrangler' freelancers won't build anything worthwhile as they're not interested in the long term unless forced.
There must be somebody you know, or friend of friend, to get a warm intro to an experienced developer you can pay to oversee said team on an interim basis. No consultants, agencies etc. they'll fleece a non-tech like you, save your money and drop it on somebody good who can actually help execute your vision.
Freelancers' careers live or die based on where future work is coming from. Happy clients are good for repeat business. Happy clients refer other potential clients. Happy clients are good for portfolios or case studies. Smart freelancers are all about keeping their clients happy in the long term. It's just good business.
Of course there are people in the freelance world who are hopeless and will never build anything useful just like there are people in the employment world with the same problem. But it's much more career-endangering to be incompetent as a developer and/or as a communicator in the freelance world because freelance gigs can often be terminated relatively easily if the client isn't happy with the work.
Maybe this is different in the parts of the US where compensation for tech employees can be way higher than anywhere else but in most of the world freelancers tend to be relatively good at what they do and part of the attraction of going the freelance route is that it doesn't have the glass ceiling on compensation that being an employee often does so if you're good enough to justify higher fees then you can actually charge what you're worth.
Not a freelancer, but this is a classic "market for lemons" situation: designing things for the long term takes a significant amount of extra thought and expertise. If two freelancers give two different quotes, one of which has a higher hourly rate and will take a longer time, there are one of two possibilities:
1. The more expensive quote is way overpriced, trying to extract money out of the buyer and possibly double-dip
2. The cheaper one is a low-quality "cowboy" job that's going to cost way more to maintain in the long run
If buyers in general can't tell the difference, then the market will end up pressuring even the high quality people to cut corners and produce "cowboy" jobs. In other words, it may not be 100% the freelancer's fault they're "not interested in the long term unless forced".
So part of the job of a "wrangler" is to understand what's worth investing in and what's not -- to know that it's worth spending double in this particular area because it's a long-term win -- and to be able to evaluate the output of the work, to make sure that what was delivered actually is higher value.
There are enough buyers who can tell the difference. Good freelancers just gravitate towards good clients and vice versa.
A pattern I commonly see is that they are heavily technology focused and don't really care about the domain, because domain-specific skills are not transferable. They are snake-oil salesmen, every problem can be solved by adopting some hot new piece of technology which can be added to their CV and increase their market value. What's frustrating is that their contribution is in the end often negative rather than merely zero - adopting over-architected solutions looking good on CV will result in great costs. If you have a strong technical leadership, you can catch their bullshit, but in the op's situation, they would be pretty much relying on freelancers' good will and abilities without much of a chance to verify their claims.
To be fair, I've met also a number of very good freelancers, usually staying with the company for years.
This same danger exists also with the normal employees, but I think this "pad the CV, move on" is more common in freelancers since they generally operate on shorter timescales.
Software needs to be designed with an understanding of the business needs and goals. You need someone who understands how to keep the software well designed (ie. Easy to debug, update and extend in the future).
Judging solely as a user of the software gives you no insight into whether or not a plate of spaghetti code could be holding behind the scenes.
Good code has two traits: It does the job it was designed to do. It is easy to update and maintain.
Everything else you read online is nonsense. And you need someone trusted inside your company to ensure the second trait is being adhered to.
Why? There are many terrible everything out there. Eventually you are always relying on the quality of your developers' output and their honesty in explaining it whether they are hired as employees or working freelance.
Crucially that is as true of the technical in-house person as the outside freelancer. I've seen scenarios with my own eyes where an experienced freelancer was better - sometimes much better - than the in-house "senior" people but the latter made critical reports about the freelancer to management. Maybe they were defensive because someone better than them was brought in. Maybe - and I suspect this is more likely in at least some of the situations I'm remembering here - the in-house person was so far below the outside freelancer in ability that they simply didn't realise how much better the freelancer's work was than their own or understand the reasons the freelancer was following certain good practices and the risks they were mitigating by doing so.
So my question to you is this: Why do you believe you can trust someone to evaluate the quality of the work more accurately and honestly just because they are inside your company?
I think there's a big difference between bringing on a boutique consulting shop and hiring freelancers who sit together in your office. The sense of ownership and professional responsibility is way higher in the latter case. In fact I'd even go as far to say that many freelancers have MORE professional pride than permanent employees, many of whom just want to draw a paycheck and live their life.
YMMV of course!