Being experienced engineers is helpful. Whether or not you're "much more involved" than recruiters probably depends on the recruiter, but running multiple interviews internally with coaches is definitely a worthwhile service.
Most experienced recruiters can rip apart a resume and are usually willing to help make changes (specifically when it's for their own client). I've seen recruiters that claim ownership of the resume they modify for their candidates, which seems tacky.
I understand that you will recommend companies that you don't work with, although your business model has all the same incentives that the traditional contingency recruiting model has.
You have two ways of making money.
1 - Getting paid when a candidate accepts a job at your client (standard contingency model).
2 - 20% (of difference) paid by candidate for negotiation
You can recommend candidates to companies you don't work with, and you can make money off these deals by providing a negotiation service.
I respect that you are trying to provide a superior service to what most recruiters are providing. It's a sad state for the recruiting industry when companies are reluctant to even use the term "recruiting". I'm as critical of the recruiting industry as anyone, so I do wish you well.